Observations of ULF wave related equatorial

Journal of Atmospheric and Solar-Terrestrial Physics 103 (2013) 157–168

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Observations of ULF wave related equatorial electrojet and density fluctuations E. Yizengaw a,n, E. Zesta b, C.M. Biouele c, M.B. Moldwin d, A. Boudouridis e, B. Damtie f, A. Mebrahtu g, F. Anad h, R.F. Pfaff i, M. Hartinger j a

Institute for Scientific Research, Boston College, Boston, USA Air Force Research Laboratory, AFRL/RVBXP, Kirtland AFB, USA c Department of Physics, University of Yaoundé I, Yaoundé, Cameroon d Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, USA e Space Science Institute, Boulder, CO, USA f Washera Geospace and Radar Science Laboratory, Bahir Dar University, Bahir Dar, Ethiopia g Department of Physics, Mekelle University, Mekelle, Ethiopia h Centre de Recherche en Astronomie Astrophysique et Géophysique, Algiers, Algérie i NASA Goddard Space Flight Center, Greenbelt, MD, USA j Institute of Geophysics and Planetary Physics, University of California, Los Angeles, USA b

art ic l e i nf o

a b s t r a c t

Article history: Received 4 August 2012 Received in revised form 6 March 2013 Accepted 11 March 2013 Available online 22 March 2013

We report on Pc5 wave related electric field and vertical drift velocity oscillations at the equator as observed by ground magnetometers for an extended period on 9 August 2008. We show that the magnetometer-estimated equatorial E B drift oscillates with the same frequency as ULF Pc5 waves, creating significant ionospheric density fluctuations. We also show ionospheric density fluctuations during the period when we observed ULF wave activity. At the same time, we detect the ULF activity on the ground using ground-based magnetometer data from the African Meridian B-field Education and Research (AMBER) and the South American Meridional B-field Array (SAMBA). From space, we use magnetic field observations from the GOES 12 and the Communication/Navigation Outage and Forecast System (C/NOFS) satellites. Upstream solar wind conditions are provided by the ACE spacecraft. We find that the wave power observed on the ground also occurs in the upstream solar wind and in the magnetosphere. All these observations demonstrate that Pc5 waves with a likely driver in the solar wind can penetrate to the equatorial ionosphere and modulate the equatorial electrodynamics. While no direct drift measurements from equatorial radars exist for the 9 August 2008 event, we used JULIA 150 km radar drift velocities observed on 2 May 2010 and found similar fluctuations with the period of 5–8 min, as a means of an independent confirmation of our magnetometer derived drift dynamics. & 2013 Elsevier Ltd. All rights reserved.

Keywords: ULF waves Pc5 pulsations Equatorial electrodynamics Electric field

1. Introduction One of the drivers of equatorial ionospheric motion is the east– west electric field through vertical E B drift, which is directly proportional to the equatorial electrojet (EEJ) (e.g., Rastogi and Klobuchar, 1990; Anderson et al., 2004). Our knowledge of the equatorial vertical drift has increased significantly over the last decades. Observation of drift velocity both from the ground (e.g., Fejer et al., 1989) and space, using instruments onboard LEO satellites, (e.g., Anderson et al., 2009; Kil et al., 2009; Yizengaw et al., 2009) as well as theoretical modeling studies (e.g., Scherliess and Fejer, 1999) are among the important factors that enhanced

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Corresponding author. Tel.: þ1 617 552 0146; fax: þ 1 617 552 2818. E-mail address: [email protected] (E. Yizengaw).

1364-6826/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jastp.2013.03.015

our understanding of the equatorial ionospheric motion and its main drivers. Additionally, the dayside vertical E B drift velocity can be estimated using a pair of magnetometer observations (e.g., Anderson et al., 2004; Yizengaw et al., 2011 and references therein). One aspect of our knowledge of the vertical E B drift that remains poorly understood is its small scale periodic fluctuations that occur quite often. Such fluctuations may reflect the fluctuation of the ionospheric layer at the equatorial region which could possibly cause the ionospheric density such as the total electron content (TEC) to fluctuate and thus trigger ionospheric irregularities and signal scintillation. There are very few observations of E B drift velocity periodic fluctuations. Patel and Lagos (1985), using Jicamarca radar, observed F-region plasma drift oscillations. For the first time, they attributed this periodic oscillation to ultra low frequency (ULF) hydrodynamic wave electric field. Almost a decade later, Reddy et al. (1994), using

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E. Yizengaw et al. / Journal of Atmospheric and Solar-Terrestrial Physics 103 (2013) 157–168

magnetometer and ionosonde observations, found a good correlation between F-region ionospheric height fluctuations and magnetometer H-component oscillations, which is directly proportional to the vertical E B drift velocity. The observed oscillations had a period of ∼8 min, which is in the range of ULF Pc5 waves. Motoba et al. (2004) also observed ULF wave related of long-period (5–15 min or 1–3 mHz) HF Doppler oscillations in the low-latitude ionosphere. Pc5 ULF waves are often observed near auroral latitudes but also can be observed at mid- and low-latitudes (e.g., Saito, 1969; Yumoto, 1985; Zhu and Kivelson, 1989; Allan and Poulter, 1992; Bloom and Singer, 1995; Motoba et al., 2002), and even equatorial latitudes (e.g., Yeoman et al., 1990; Reddy et al., 1994) and have been extensively studied for many decades as a good diagnostic tool of the magnetosphere and ionosphere interactions. There is evidence that ULF waves also penetrate to equatorial latitude and different penetration mechanisms have been suggested (Kikuchi and Araki, 1979b; Chi et al., 2001). According to one penetration mechanism it is through a combination of fast and shear Alfvén waves that the ULF wave directly penetrates from the magnetosphere to low latitudes (Chi et al., 2001), while Kikuchi and Araki (1979b) argued that the ULF wave's energy arrive from the magnetosphere to high latitudes first and then propagates to low latitudes through an ionosphere waveguide. Since the objective of this paper is to address the ULF oscillation in the EEJ and the vertical drifts, we do not concern ourselves with the mechanism by which the ULF waves propagate to the equatorial region but simply accepted as a fact. We will therefore not be addressing the controversy presented by the two publication mentioned above. The penetrated Pc5 wave can then modulate the equatorial electrojet, in the same manner Pc5 waves modulate the auroral currents. This may cause the vertical fluctuations of the F-region ionospheric density that may, in turn, trigger ionospheric irregularities and scintillation (e.g., Reddy et al., 1994). In this context, we report in the present study, the Pc5 ULF wave related vertical E B drift fluctuations observed at the equatorial region on 9 August 2008 and 2 May 2010. We estimate the dayside vertical E B drift velocity at the equator using data from a pair of magnetometers (one at the equator and one off the equator by 6–91 geomagnetic) based on the technique described by Anderson et al. (2004). For the 2 May 2010 event, we found similar ULF type fluctuations in radar measured vertical drift velocity. At the same time, the Global Positioning System (GPS) total electron content (TEC) shows similar fluctuation with a period of 8–10 min, indicating that the ULF waves can indeed trigger ionospheric density fluctuations. Finally, we use observations from solar wind monitors and magnetospheric satellites along with ground observations and found that the ULF wave oscillations are likely driven by similar upstream waves. Therefore, the prime objective of the paper is to report the ULF wave related fluctuations that were observed simultaneously at the equator using different instruments, such as GPS TEC, magnetometers, and radar observation vertical drift velocity, which is the first observation of such an occurrence.

ACE spacecraft. At the time of the event, ACE was located at (233, 40, 2.5) RE in GSM coordinates. The dayside magnitude and direction of the E B drift in the African sector is estimated using the AMBER ground-based magnetometer located in Adigrat and the INTERMAGNET magnetometer in Addis Ababa (see Table 1). In the American sector we used the magnetometers located at Jicamarca and Piura (see Table 1). According to the technique by Anderson et al. (2004) the horizontal magnetic field perturbation at the magnetic dip equator is the combined result from the EEJ and magnetospheric and other external currents such as penetrating, from highlatitudes, ionospheric currents. This produces strong enhancement in the H component magnetic field observed by magnetometers within 731 of the magnetic equator. The geomagnetic field perturbation outside the equatorial region, at ∼61–91 geomagnetic, is the result of all other currents with only no response to the EEJ. Thus, when the H component observations from a magnetometer at ∼61–91 geomagnetic are subtracted from the H component values measured by a magnetometer at the magnetic equator, the difference is only related to the EEJ contribution, which is directly related to the eastward electrostatic field that created the electrojet current and hence proportional to the vertical E B drift (e.g., Anderson et al., 2004; Yizengaw et al., 2004). The magnitude and direction of the E B drift is then estimated using these ΔH values according to the formulas in Anderson et al. (2004). Pc5 waves in the magnetosphere were detected by the magnetometer onboard GOES 12, located at ∼75.01W longitude, and by the VEFI fluxgate magnetometer on board the C/NOFS satellite. C/NOFS has an 850 by 400 km elliptical orbit with 131 inclination.

2. Space- and ground-based data sets

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On 9 August, 2008 the high density, high ram pressure region that precedes a solar wind fast speed stream and a co-rotating interaction region (CIR) impacted the magnetosphere between 0000 and 1000 UT. Such magnetic activity could be characterized as a weak storm. The time period we examined has high dynamic pressure and fluctuating (mostly positive) IMF Bz. The solar wind parameters are measured by the SWE instrument (Ogilvie et al., 1995), while the interplanetary magnetic field (IMF) was measured by the magnetic field experiment (Lepping et al., 1995) onboard

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Table 1 List of magnetometer stations used. Station Name

Station Code

Project it belongs

Adigrat Algeria Addis Ababa Jicamarca Piura Putre

ETHI ALGR AAE

AMBER 14.31N AMBER 36.91N INTERMAGNET 9.01N

JIC PIU PUT

Geog. Geog. Geom. Geom. Latitude Longitude Latitude Longitude

11.791S 5.21S 18.31S

SAMBA

39.51E 2.91E 38.81E

6.01N 28.01N 0.91N

111.11E 77.71E 110.51E

77.161W 80.631W 69.51W

0.751N 6.821N 5.51S

5.671W 9.391W 1.41E

40 20 0

-150

-100

-50

0

50

100

Fig. 1. The geographic location of the ground-based magnetometers and GPS receiver used for this study. The blue and black semi-vertical curves indicate the geomagnetic field lines at 0.01, 80.01, and 110.01 E. The solid horizontal line depicts the geomagnetic equator, and the two dashed lines denote the Equatorial Electrojet (EEJ) region. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)


20 Frequency (mHz)

The Pc5 wave activity was also recorded by a number of ground magnetometers, but with different amplitudes at different latitudes. Data from the AMBER magnetometers in the African sector (Yizengaw and Moldwin, 2009) and SAMBA magnetometer in the American sector are used. The sampling rates of AMBER and SAMBA magnetometers are half and one second, respectively. Fig. 1 shows the geographic location of the ground-based instruments used for this study, color-coded for the different chains, while their geographic and geomagnetic coordinates are recorded in Table 1.

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15 10 5 0

3. Data analysis and observations 3.1. Equatorial fluctuations Fig. 2 shows the ACE propagated solar wind properties according to Weimer et al. (2003), in GSM coordinates, along with geomagnetic indices during the CIR of August 9–10, 2008. We show from top to bottom the solar wind speed (VSW), number density (NP), the IMF B-field magnitude (|B|), IMF z-component (BZ), ram pressure (PRAM) in black and AE index in red, and Dst and Kp (black and green bars indicate quiet and disturbed condition) indices. During the 3-day interval of Fig. 2 the solar wind velocity gradually increased from ∼320 km/s to 670 km/s, while the ram pressure, proton density, and field magnitude show a strong enhancement between 0000 and 1000 UT on 9 August, 2008. IMF BZ shows oscillatory behavior during that time, which is

Fig. 3. Equatorial electrodynamics perturbation: high pass filtered of E B drift velocity along with its dynamic spectra plotted as a background (top panel) and the magnitude of E B drift velocity (bottom panel). (LT¼ UT þ3).

usually attributed to the effects of interplanetary Alfvén waves (e.g., Tsurutani et al., 2006), typically observed during CIRs-driven storms. The Dst index shows a gradual positive increase starting from 0000 UT and reaches a maximum value (∼40 nT) at 0500 UT on 9 August, in phase with the IMF |B| and ram pressure enhancements. There is no storm sudden commencement (SSC) before the storm main phase, rather the Dst index shows a gradual rise, indicating that for this specific storm no fast-forward shock formed. This is a weak storm with a minimum value of Dst of −41 nT. The bottom panel in Fig. 3 shows the E B drift estimated from ground-based magnetometer data. The AMBER magnetometer in Adigrat and the INTERMAGNET station in Addis Ababa, Ethiopia are used to estimate the EEJ or E B drift. The E B drift velocity shows large scale fluctuations during the 0300–1200 UT period (0600–1500 LT). The nearly periodic higher frequency (a few minutes) small scale drift velocity fluctuations are embedded into the large scale fluctuations (an hour). In order to see the type of wave embedded into this small scale fluctuation, a band-bass filter has been applied to the E B drift velocity and is plotted as a white line in the top panel of Fig. 3 along with the dynamic spectrum of the same E B wave in the 0–20 mHz frequency range. The filtered E B drift velocity (top panel) has a clear periodic oscillation with a period of 5–8 min, typical of Pc5 waves. The dynamic spectrum also shows a clear band of waves in the range of 1–6 mHz from 04–11 UT. At 0620–0645 UT (0920–0945 LT) there is a short-lived, wide band, wave power burst that extends into the Pc4 range and above (f 410 mHz). Detailed description of the dynamic spectra analysis that we applied for this study can be found in (Boudouridis and Zesta, 2007). 3.2. Ground magnetometer fluctuations Fig. 2. Variation of solar wind parameters and geomagnetic indices: solar wind speed, number density, IMF B-field magnitude, Bz-component, ram pressure (black) and AE index (red), Dst (blue curve) and Kp (bars) indices are shown from top to bottom panels, respectively. The vertical dashed lines represent the time when the storm starts. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Over the following three figures we examine filtered time series and dynamic spectra of magnetometer data from three key ground stations, ETHI, ALGR, and PUT. The time period 0125– 1000 UT is plotted in each case. ETHI and ALGR in the African

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sector go through the dayside 0425–1300 LT and 0225–1100 LT sector, respectively, during that UT interval, while PUT in the American sector spans the 2155–0530 LT nightside sector. Fig. 4 shows from bottom to top the dynamic spectra and band-pass (0–20 mHz) filtered time series data of the northward (BX), eastward (BY), and vertical (BZ) magnetic field components recorded at the AMBER station ETHI (61N geomagnetic) between 0125 and 1000 UT (0425 and 1300 LT) on 9 August 2008. During that time period high ram pressure and magnetic field strength was impacting the magnetosphere. The white curve over-plotted in the first, third, and fifth panels from bottom to top depicts the propagated solar wind number density from the ACE spacecraft that varies in complete phase with the ram pressure. The solar wind number density value range is shown at the right side of the bottom panel. The linear color scale of wave amplitude for all dynamic spectra is shown on the right side of the third panel from the bottom. The wave is significantly stronger in the north-south (BX) component than in the east-west (BY) component and is evident both in the filtered time series and in the dynamic spectrum of that component. Most of the wave power is confined between 1.0 and 7 mHz, which is in the Pc5 wave band, with a peak at ∼1–2 mHz. No significant wave signature is observed in the vertical (BZ) component. The solar wind density plotted in Fig. 2 is propagated to the bow shock (Weimer et al., 2003). An additional delay of 2 min has been added to those times to account for propagation from the bow shock to the ground. This is only a rough estimate and we

Fig. 5. Similar to Fig. 4, but for the magnetometer data from Algiers, Algeria (LT¼ UT þ1).

make no attempt to rigorously calculate the wave propagation through the magnetosheath, the magnetosphere and down to the high and low-latitude ionosphere, since it is irrelevant to the cross correlations we demonstrate later. The Pc5 wave signature observed at ALGR (28.01N geomagnetic) is shown in Fig. 5 in the exact same format of Fig. 4, and was strong in both the north–south and east–west components, although the frequency peak in the Bx are similar between the two stations (ALGR and ETHI). ALGR is located in a meridian close to that of ETHI (just 351 to the west), but is at higher latitude close to the equatorial anomaly peak region. There is also a reasonably good correlation between the solar wind number density peaks and the Pc5 wave bursts seen in the x-component dynamic spectra of both Figs. 4 and 5. Further correlation analysis is given in Section 3.4. Fig. 6 is in the identical format as Fig. 4, but presents nightside Pc5 pulsation activity on the ground observed by the SAMBA magnetometer PUT. The Pc5 pulsation observed at PUT was stronger in the BX component while very little power was observed in the BY and BZ components. For the time series plots shown in the figure, the solar wind number density perturbation and Pc5 bursts have reasonable temporal agreement. 3.3. Fluctuations in the magnetosphere and ionosphere Fig. 4. The Fourier spectra of a ground-based magnetometer data recorded on 9 August 2008 using AMBER magnetometer network located at Adigrat, Ethiopia. From bottom to top, the figure shows the dynamic spectra and high pass filtered Pc5 pulsation of BX, BY, and BZ. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

In order to see ULF wave signature in space, especially in which component of the field it gets stronger, we analyzed magnetometer data from GOES 12 observation. During the time interval mentioned above (i.e., 0125–1000 UT), GOES 12 was in the nightside. The GOES


Fig. 6. Similar to Fig. 4, but for the data recorded in the American sector using a SAMBA magnetometer located at Putre, Chile (LT¼ UT−4.5).

12 foot-point goes through local midnight at ∼0530 UT, and thus GOES was between 1900–0430 MLT during the time interval between 0125 and 1000 UT. Fig. 7 shows GOES 12 observations in the exact same format as the previous three figures and for the same time period. From bottom to top we plot the dynamic spectra and band-pass-filter time series of the BP, BN, and BE components of the magnetic field observed by GOES 12 magnetometer. At the spacecraft location Bp is perpendicular to the satellite orbital plane or parallel to the Earth's spin axis, BE is perpendicular to BP and is directed earthwards, and BN is perpendicular to both BP and BE and is directed eastwards. Therefore, for our data analysis, we consider Bp as north–south (parallel to B), BN as east–west (azimuthal), and BE as up–down (radial) components of the magnetic field at the spacecraft location. The ULF wave power was dominant in the azimuthal component and with significantly less power seen in the radial component, and almost no wave power in the compressional (field-aligned) component. However, on the ground at PUT, which was also located in the night side (see Fig. 6) during the time of interest (0125–1000 UT), the Pc5 wave was stronger in the north– south component instead of azimuthal (east–west) component. Because of the 901 rotation, applied by the ionosphere, the stronger azimuthal oscillation in space (e.g., at GOES 12 location) would manifest as north–south oscillations on the ground (e.g., Hughes and Southwood, 1976), so there seems to be good agreement between the type of ULF waves observed between GOES and PUT, both supporting primarily toroidal mode waves. Although the theory of Hughes and Southwood (1976) should be applied to the high latitude where the Hall current plays a major role in the distribution of the ground magnetic field, the 90 degree rotated Pc5

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wave signature at high latitude can penetrate to low-to-equatorial latitude and modulate the electrojet at the equator. The propagated solar wind number density from the ACE spacecraft is again overplotted onto the dynamic spectra plots. The stronger ULF wave activity (between 0300–0630 UT) coincides well with the solar wind density peaks, and individual wave bursts seem to correlate with individual density peaks. The wave frequency seems to peak at ∼1–2 mHz (same as in the three ground stations), while several harmonics at higher frequencies are also observed. One THEMIS spacecraft (Th-E), located at lower latitudes in the dayside sector (1500–1640 MLT or 0500–0730 UT), detected Pc5 pulsations (not shown here) that terminated just before 0700 UT, same as on the ground and on GOES. For further supporting observations, also not included in this manuscript for brevity, we note that the Pc5 observations were observed on the dayside at all latitudes from equatorial to high latitudes with the exact same period. So, summarizing the ground and magnetospheric observations, Pc5 waves were observed both on the dayside and nightside in the magnetosphere and at all latitudes in the ionosphere at the same time period as similar oscillations were observed in the solar wind and there is some generally good correlation between peaks in the solar wind and bursts of enhanced Pc5 activity both in the magnetosphere and on the ground in the 40–60 min time scale. Correlations in a higher frequency range are explored in a more rigorous way below. We therefore propose that the Pc5 wave observed in the equatorial ionosphere is likely driven by similar oscillations in the upstream solar wind and are similarly present in all regions of the magnetosphere and ionosphere. We also inspect C/NOFS magnetometer observations to see the ULF wave signature at ionospheric altitudes. Fig. 8 shows the Pc5 pulsation observed by C/NOFS during its multiple passes at equatorial latitudes in the ionospheric F-region altitudes. The Fourier window used for the dynamic spectra of Fig. 8 is 20 min (as opposed to the 40 min window used for all other observations) due to the fast passing of C/NOFS through ionospheric structures. In the given example, there are five passes with a period of about 100 min. On the top panel the red curves are the C/NOFS ground tracks and the blue curves are the satellite altitude. The x-axis is geographic longitude for the five overlapping passes, 0125–0309 UT, 0309–0454 UT, 0454–0637 UT, 0637–0820 UT, and 0820–1000 UT, which are marked as orbit 1, 2, 3, 4, and 5 in the figure. The black line is the dip equator. Panel 2 from top shows the local time coverage of each C/NOFS's pass. In panels 3 through 8 we plot the band-pass filtered time series and dynamic spectra of the three components of the magnetic field. The VEFI fluxgate magnetometer uses the local east, north, and up (ENU) coordinate system, and by convention the east-west axis is labeled as X-component, north–south as Y-component, and up–down as Z-component. In order to be consistent with the ground-based magnetometer data, we transform the ENU to geographic and then to geomagnetic coordinate system at C/NOFS's location. However, in the geomagnetic coordinate system, Z-axis points along the dipole axis and the X- and Y- axes point to Sun–Earth line (vertical) and azimuthal (east). Therefore, in Fig. 8, Bz is the north–south (parallel to B or compressional), By the east–west (azimuthal or toroidal), and Bx the up–down component of the field at the C/NOFS location. The sections between each orange and its following white vertical dashed lines in the dynamics spectra panels, and orange and blue in panel 2 from top, depict the time zone when C/NOFS was below 500 km. The solar wind number density is similarly over-plotted in the dynamic spectra panels. The satellite observed strong amplitude Pc5 waves in both compressional and toroidal components (Z and Y) at all local times and at all altitudes of the C/NOFS orbit, indicating the Pc5 signature was not only a dayside phenomenon, but also occurs on the nightside. The Pc5 wave power was significantly visible on

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Frequency (mHz) Pc5 - Be or U-D(nT) Frequency (mHz) Pc5 - Bp or N-S(nT) Frequency (mHz)Pc5 - Bn or E-W(nT)

Fourier spectra, GOES 12, 9 August 2008 (day 222) 6 4 2 0 -2 -4 -6 20

6 4 2 0 -2 -4 -6 20

15 10

0.6 0.5 0.4 0.3 0.2 0.1 0.0

15 10 5

Wave amplitude

5

6 4 2 0 -2 -4 -6 20 15 10 5 0200

0300

0400

0500 0600 0700 Universal Time

0800

0900

1000

Fig. 7. The Fourier spectra of a magnetometer data recorded at the geosynchronous orbit by GOES 12 spacecraft on 9 August 2008. From top to bottom, the figure shows the high pass filtered Pc5 pulsation and dynamic spectra of BE, BN, and BP field components (LT ¼UT−5.0).

both components, dominantly on the north–south and east–west components. Unlike the ground- and space-based, at GOES, observations, the Pc5 power signatures observed at C/NOFS are more wide-band (1–8 mHz) and more continuous, and manifest in both the toroidal and compressional modes. This is not wholly unexpected. Pc4-5 waves are very long wavelength structures, of the order of multiple RE, so a low-Earth orbit satellite like C/NOFS would cross the wave structures multiple times over multiple orbits in the period plotted and as the wave structures enhance or change in time. So the spatial and temporal aspect is heavily merged in the C/NOFS observations. The untangling of spatial and temporal variations is not within the scope of the present paper, nevertheless, Fig. 8 does offer strong support to the hypothesis that the Pc4-5 waves are global. The Pc 4-5 waves are observed at all altitudes of the satellite orbit (∼400–800 km) and there is reasonable agreement between the solar wind number density 30–50 min perturbation peaks and the occurrence of strong Pc5

burst in the dynamic spectra, especially in the Y (toroidal) component. That implies that these are temporal wave power enhancements that C/NOFS observes no matter where it is located at the moment. In summary, it appears that wave bursts in the Pc5 frequency range appear in general agreement with fluctuations in the solar wind dynamic pressure. Further analysis from space and ground magnetometers shows that the Pc5 wave bursts are primarily toroidal oscillations that appear to be global, seen both on the nightside and dayside. The ULF wave observations both on the ground and in space appears to be in the frequency range of 1–7 mHz which is in the Pc5 range. The Pc5 wave dominated at the east-west component in space but on the ground it became stronger at the north–south component. This is believed to be the 901 rotation of the wave introduced by the ionosphere when it penetrates to the ground from the magnetosphere (e.g., Hughes and Southwood, 1976).


Fig. 8. Similar to Fig. 6, but for the C/NOFS satellite magnetometer data. From top to bottom, the figure shows the ground tracks (red curves) and altitude (blue curves) of the satellite that are marked as orbit 1, 2, 3, 4, and 5, local time coverage of each pass, high pass filtered Pc5 pulsation and dynamic spectra of north–south, east– west, and up–down field components. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

3.4. Solar wind driver First we demonstrate that the solar wind, magnetosphere, and ionosphere contain nearly the same type of oscillations during the period of time studied. Fig. 9 shows the power spectra during the period of time studied (black curve) and a day prior to the event (blue curve) for the solar wind number density (top left), the east– west component of the magnetic field at GOES 12 (top middle) and at C/NOFS (top right), and for the north–south components of the field on the ground at Ethiopia (bottom left), Algeria (bottom middle), and Putre (bottom right). As can be seen in the figure, the frequencies of higher power spectrum peaks (strongest oscillations) of both the source and ground- and space-based observations are almost on the same frequency range, i.e., in the vicinity of 1.0–1.5 mHz. This confirms that the Pc5 wave that we saw in the space-based observation (Figs. 7 and 8) is rotated by 90 degree and seen in the north–south component of the ground-based observation (Figs. 4–6). We also note the frequency similarity between the power spectrum peaks at PUT (located at the night side in the

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South America sector) and ETHI (located at the dayside in the African sector). On the other hand during the day prior to the storm day, when there is no Pc4-5 wave power in the solar wind no such wave power appears in either space- or ground-based observations, as shown by the blue curve in Fig. 9. Incidentally, the power spectra on the day after, Aug 10, are very similar to those of Aug 8, namely no Pc4-5 wave power is present in the solar wind nor in the magnetosphere or on the ground. This clearly shows that the Pc5 waves observed by both space- and ground-based magnetometers were a global event that was likely driven by the solar wind dynamics. The wave power spectrum peaks at C/NOFS do not agree very well with the rest as shown in Fig. 9. This could be due partly to a Doppler shift caused by the spacecraft's higher velocity and the different Alfven speed in this low altitude orbit (e.g., Streltsov and Lotko, 2003). Fig. 10 shows the cross correlation between the power in solar wind dynamic pressure (Psw) and the Bx component at three ground magnetometer stations, ALGR and ETHI in Africa, and PUT in South America for 9 August, 2008. From the bottom, the figure shows the dynamic spectra of solar wind pressure, Bx component at ALGR, ETHI, and PUT, as a function of universal time and frequency in the range 0.3–8.3 mHz. Each image is scaled according to the color scale on the right. The total powers in nPa2 and nT2 are over plotted on the spectra for two frequency ranges, 0.3–9.0 mHz in red (essentially the entire frequency range available for the 1 min resolution pressure and field data used), and 0.5–1.5 mHz in blue where most of the power appears, according to the power spectra of Fig. 9. The powers are scaled according to the red axis on the left of each image. In order to achieve higher frequency resolution, the dynamic spectra are produced using a 60 min data window for the Fourier transform centered on each 1 min data point, which is then shifted by one minute to produce the spectrum for the next data point. The second panel from top shows the correlation coefficient (0–1) for the cross correlation between the Psw power and the three ground stations Bx component power in the 0.5–1.5 mHz frequency range, with Psw/ALGR (black), Psw/ETHI (blue), and Psw/ PUT (red). At each power data point, 1 h from the beginning of the dynamic spectra to 1 h before the end, we calculate the maximum correlation coefficient within a 2-hour power window centered at that power data point. The Psw and Bx 2 h long power data segments are shifted with respect to each other (with time lags in the range of 0–60 min) to obtain a correlation curve. The maximum of this curve is plotted in this panel. The correlation window is then shifted by 1 min and the whole process is repeated to produce the three correlation coefficients with 1 min resolution. The average correlation coefficients for all three ground stations are greater than 0.5. In particular the correlation between Psw and Bx for the dayside equatorial station, ETHI, is significantly high with average correlation coefficient of 0.75. This clearly shows that the Pc5 wave that we observed both in space and on the ground was more likely a solar wind driven ULF wave. The top panel shows the correlation time lag, the time in minutes that the solar wind pressure needs to be shifted forward in time for the respective correlation coefficient between Psw and Bx to obtain the maximum value reported above. This time lag measures the time delay between the Psw power input and the corresponding ground Bx power response, with values ranging from 0–10 min, indicating a prompt response of the ground magnetometer wave power to the likely solar wind dynamic pressure driver. In summary, while Fig. 9 demonstrated that solar wind, magnetospheric and ground monitors, all observe nearly the same wave power content during the same time period of interest, Fig. 10 goes one step further and demonstrates that the actual bursts of the identified wave power (centered at 0.5–1.5 mHz) are

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Fig. 9. Power spectra for solar wind number density and for north–south field component of ground-based observation and for east–west component of the space-based (GOES and C/NOFS) observations. The vertical dashed lines are drawn just see those matching peaks. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

well correlated between the solar wind and the ground with the ground power bursts lagging by 1–10 min.

4. Discussion and conclusion With observations from the ACE spacecraft, we show direct evidence that solar wind number density and ram pressure fluctuations observed upstream of the terrestrial magnetosphere, generate Pc5 ULF waves in the magnetosphere. Specifically, we show that the same Pc5 waves are observed in the magnetosphere by the geosynchronous GOES spacecraft, at ionospheric altitudes by the C/NOFS satellite, and on the ground by ground-based magnetometers. This has been demonstrated by showing (see Fig. 9) that the strong power spectral peaks (strongest oscillations) appear in the vicinity of the same frequency both at the source, and ground- and space-based observations. The spectral peaks correlation between the source and ground-based observations also shows (see Fig. 10) very good correlation coefficient. The periodic solar wind dynamic pressure oscillations slowly alter the size of the magnetospheric cavity, causing the magnetosphere to increase or decrease in size as needed to balance the dynamic pressure. When the external driver ceases, the compressional oscillations in the magnetosphere, as well as their signature in the ionosphere stop. Kepko and Spence (2003) studied a similar case and found no evidence of the continuation of compressional magnetospheric oscillations once the solar wind driver stopped. Our observations also support this. The Pc5 pulsations observed by magnetometers both on the ground and in space suddenly discontinue just before 0700 UT on 9 August 2008 (see Figs. 4–7). The periodic fluctuation of solar wind ram pressure or number

density (white curves on the spectra plot in Figs. 4–7) also disappears just before 0700 UT. However, while it terminated just after 0700 UT in the north–south and up–down component, the Pc5 wave did not terminate in the east–west component on C/NOFS's observations (see in Fig. 8). This happened when C/NOFS was above 500 km altitude, indicating that those waves, especially the fast mode wave (e.g., Kikuchi and Araki, 1979a), may be confined in the F-region and not has penetrated in the E-region, which is why the ground-based observations did not see it. The simultaneous termination of Pc5 pulsations both on the ground (Figs. 4–6) and in the magnetosphere (Fig. 7) indicates that the ULF wave did indeed penetrate into the ionosphere from the source region, namely, the upstream solar wind coupling with the magnetosphere. The interesting question that is worthy to mention here is, why the ULF wave signature ceased both in the magnetosphere (at GOES location) and on the ground but not at C/NOFS location? Is there other mechanism that can generate ULF wave in the ionosphere that may be confined in the ionosphere? Sutcliffe and Lühr (2010) suggested that ULF wave can be generated by the cavity mode type of resonance excited in the inner-magnetosphere bounded below by the ionosphere and at high altitudes by an Alfvén velocity gradient. However, these questions are beyond the scope of this paper to address or confirm, and thus further investigations are required to answer all these interesting problems. The Pc5 pulsation that we present here occurred in the upstream solar wind number density and ram pressure. In this study, we demonstrated that when the magnetospheric driven Pc5 oscillation penetrates to equatorial latitudes it can modulate equatorial electrodynamics. Fig. 3 illustrates this by showing the driven oscillations of the E B drift estimated from ground-based magnetometer data. The filtered E B drift shows similar Pc5-range


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Psw/ALGR Psw/ETHI Psw/PUT

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Psw/Ground mag power correlations, 09/08/2008 (222) 10 8 6 4 2 0

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10

Fig. 10. From the bottom the figure shows the dynamic spectra of solar wind pressure, Bx component at ALGR, ETHI, and PUT, as a function of universal time and frequency. The second panel from the top shows the correlation coefficient for the cross correlation between the Psw power and the Bx component power at three ground stations with Psw/ALGR (black), Psw/ETHI (blue), and Psw/PUT (red). The top panel presents the correlation time lag that measures the time delay between the solar wind pressure power input and the corresponding power on the ground-based Bx component. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

oscillations as were observed by the magnetometers on the ground and in space. This indicates that the magnetospheric ULF wave, driven by the solar wind ram pressure variations, can penetrate to the equatorial region and modulate the magnetic field and hence the electric field and the vertical E B drift. We applied the same analysis technique to GPS TEC, which was calculated using the technique described in (Yizengaw et al., 2004), available in the east African meridian. The averaging of TEC introduced by merging all individual TEC time series segments (obtained from different GPS satellite passes) together precludes the identification of small scale TEC perturbations. To avoid this we applied the same band-pass filter that we applied for the magnetometer and E B drift velocity data to the individual TEC segment obtained by each satellite-receiver TEC time series. A typical TEC time series for PRN 20, observed at MALI (40.191E, 3.01S geographic, 12.421S geomagnetic) GPS station, is shown in

Fig. 11 second from the top panel, and its corresponding filtered time series and dynamic spectra are shown at the top panel, for the time period 0415–0725 UT. Although the TEC time series shows identifiable oscillations between 0500–0700 UT (0800– 1000 LT), it is very hard to identify what type of wave is embedded in it. The filtered time series and dynamic spectra, on the other hand, clearly shows a well defined periodic fluctuation with a period of 3–10 min (1–5 mHz), which is the same as ULF wave period in the range of Pc5. We also investigated a phase correlation between the TEC and vertical drift velocity. Fig. 12 presents the time series plot of unfiltered TEC (top panel) and drift velocity (blue curve in the bottom panel), showing a reasonably clear phase alignment. The red curve in the bottom panel represent the TEC over plotted with the drift velocity clearly show the phase alignment of the two parameters, especially those sharp upward drift mostly aligned

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Fig. 11. ULF related density fluctuations along with the corresponding vertical E B drift velocity for 9 August 2008. From top to bottom: the dynamic spectra as a background and filtered GPS TEC for a single satellite pass (PRN 20), GPS TEC time series, dynamic spectra as a background and filtered E B drift velocity, and the magnitude of E B drift velocity during the corresponding time slot. (LT ¼ UTþ 3). (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

Fig. 12. Time series plot of unfiltered TEC shown in the top panel. Drift velocity (blue curve), over plotted TEC (red curve), and a backward time (∼27 min) shifted TEC curve (black curve). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

with the TEC peaks as it is depicted by the vertical dashed lines that show the peak alignment of red and blue curves. These demonstrate that the ULF wave modulated vertical drift velocity

can indeed cause small scale density fluctuation as it is shown in both filtered (Fig. 11) and unfiltered (Fig. 12) time series plots. Furthermore, in order to investigate the time gap between the vertical drift velocity and the ionospheric density response, especially on those large scale structures observed on both drift and density measurements, we applied a backward (∼27 min) time shift on the TEC curve, which is shown in the bottom (black curve). The large scale structure of the TEC (black curve) and drift velocity (blue curve) have a good phase agreement, indicating the time delay of the density response to the small scale vertical drift fluctuation. We also carefully checked if the density fluctuation shown in Fig. 11 is due to shorter period gravity wave (GW) activity instead of ULF waves penetrating to equatorial latitudes and driven by similar wave activity in the solar wind. The GW, which can be generated either in the ionosphere by auroral zone disturbed condition or the lower thermosphere by tropospheric disturbances, can propagate to the ionospheric altitude and push and pull plasma along the Earth's magnetic field lines causing periodic advection and compression of the plasma called traveling ionospheric disturbances (TIDs) (e.g., Hocke and Schlegel, 1996). Auroral zone currents were quiet on 8 and 9 August 2008 until 1700 UT, as seen in the AE index of Fig. 2. AE increases after 12 UT on 9 August 2008 and soon reaches over 500 nT (Fig. 2). This implies that it is unlikely that GW would be generated at the ionospheric altitude due to auroral disturbances during that period of time. The only possibility remaining is that GW could be generated in the lower thermosphere and propagates upward to ionospheric altitude. However, when TIDs are generated from upward propagation of GWs their period in the ionosphere is much longer than 20 min because the shorter periods of o20 min (o0.8 mHz) typically dissipate in the thermosphere before they reach ionospheric altitudes due to dissipative processes such as thermal diffusivity, kinematic viscosity, ion drag, wave-induced diffusion, and nonlinear wave interactions (e.g., Richmond, 1978; Vadas and Fritts, 2005). Therefore, it safe to conclude that the density fluctuation, with a period of 4–10 min that shown in Fig. 11 is due to the ULF wave penetration to the equatorial ionosphere not due to the shorter period GW activities. The third and fourth from the top panels in Fig. 11 reproduce the E B drift filtered series and dynamic spectrum and the E B drift, shown in Fig. 3. It is noteworthy from comparing the dynamic spectra on the top and third panels that both the TEC and derived E B drift exhibit their strongest Pc5 power between 0520–0640 UT (0820–0940 LT). It is also the time range when both ground- and space-based magnetometer observed clear and strong Pc5 signatures (see Figs. 4–8). This is direct proof that the source of the TEC fluctuations is the magnetospheric Pc5 wave, which is in turn directly driven by upstream waves. The Pc5 ULF waves can penetrate into equatorial latitudes and modulate the electrodynamics and thus create ionospheric density perturbations (up to 0.4 TECU from peak to peak) which are in fact stronger than the density perturbation (up to 0.08 TECU from peak to peak) reported due to earthquakes (e.g., Calais and Minster, 1998). The 0.4 TECU peak to peak TEC perturbations can cause up to 10.7 cm range error for L2 band frequency which is equivalent to the range error caused by a TECU on the GPS dual frequencies (L1 and L2) communication navigation systems. This is in agreement with the report by Reddy et al. (1994), who unambiguously showed that the east–west electric field oscillations in the equatorial F- and E-regions are associated with a micro-pulsation event and not a purely radial (vertically up and down) electric field oscillation. Incidentally, the E B drift velocity shown in Fig. 3 reversed its direction from upward to downward right in the middle of dayside (middle of the plot). The magnitude of its fluctuations reduced significantly after the reversal of E B drift velocity (detail


explanation about the dayside E B drift reversal can be found in Yizengaw et al. (2011)). During the time when E B drift reverse its direction, the ULF wave oscillations terminated both in spaceand on the ground stations (Figs. 4–8), which is likely due to the significant weakening of the external driver, mentioned above. Unfortunately there are no other independent ground-based vertical drift observations available in the African sector to compare with our derived E B drift for this particular event. We can however demonstrate that the fluctuations seen in the magnetometer-derived vertical drift are also independently measured by the JULIA radar during other similar type of events. Fig. 13 shows magnetometer estimated E B drift (second panel from top) and JULIA 150 km vertical drift (bottom panel) velocities for 2 May, 2010. We used the magnetometers at Jicamarca and Piura to estimate the E B drift. Both the magnetometer estimated and radar observed vertical drift velocities show large scale fluctuation almost throughout the day in very good correlation. Higher frequency waves are distinguishable overlayed on the large scale fluctuations. The top and third, from top panels, show the filtered time series of drifts determined by ground magnetometers and JULIA, respectively. Both show periodic oscillations with a period of 5–10 min, which is again in the Pc5 range. The ground-based magnetometer located at Putre in Chile, which is in the same meridian as Jicamarca, also shows (not shown here) very strong Pc5 ULF wave signatures in the same frequency at the same time.

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The comparison shown in Fig. 13 provides confidence in our conclusion that the Pc5 oscillations in the E B drift derived by ground magnetometers are indeed an ionospheric density feature and can be independently observed by radar. In conclusion, we found that the Pc5 waves observed by both space- and ground-based magnetometers were a global event. We have shown, using both ground- and space-based independent observations, that ULF waves in the Pc5 range can penetrate to the equatorial ionosphere and modulate equatorial electrodynamics. This can be due to the ULF wave modulation of the magnetic and electric fields in the same manner Pc5 waves modulate the auroral currents and thus electric fields, because the ULF wave is the type of wave that propagates with modulated magnetic and electric fields. The modulation of magnetic and electric fields at the equator can significantly modulate the vertical drift which is proportional to both electric and magnetic fields. Therefore, we suggest that when ULF waves penetrate to the equator, it causes the east–west electric field and magnetic field to fluctuate with the same frequency range of ULF waves. The fluctuating east–west electric field in turn produces a fluctuating vertical drift velocity (E B drift) as has been shown by two independent drift observations that were fluctuating with identical periodicity (see Figs. 3 and 13). This in turn causes the up and down ionospheric height variation and produce a significant ionospheric density fluctuation that can create a range error of 10.7 cm (see Fig. 11) or more, depending on the strength of ULF activities. Such ionospheric density fluctuation (irregularities) can easily disrupt navigation and communication systems, especially for single frequency users. However, it is not yet well understood whether the large scale (very significant in magnitude (see Figs. 3 and 13) dayside drift velocity fluctuations, which we often observed whenever we detected ULF wave signature, have any connection with the Pc5 ULF wave activities. In order to understand all these, we believe more studies that are supported with modeling investigations are highly required.

Acknowledgment This work was supported by NASA IHY and Geospace Science Programs (NNX07AM22G and NNX09AR84G), NASA LWS programs (NNX10AQ53G and NNX11AP02G), and AFOSR YIP Grant (FA9550-10-1-0096). MBM was partially support by a NASA Geospace Grant (NNX09AI62G). The geomagnetic indexes (Dst, Kp) and solar wind data are obtained from World Data Center. The authors are indebted to the ACE, GOES, THEMIS, and INTERMAGNET team for the data resources they made available to the public. We also thank Dr. James Weygand for the propagated solar wind data.

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Fig. 13. Identical equatorial electrodynamics perturbation as observed by two independent instruments. The filtered Pc5 wave signature and the time series plots of the E B drift velocity estimated using magnetometer observation (top two panels), and that of the bottom two panels shows for drift velocity observed by JULIA 150 km radar on 2 May 2010.

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