JOURNAL OF GEOPHYSICAL RESEARCH, VOL WI, NO. A12, PAGES 26,759-26,767, ... S. A. Pulinets and K. F. Yudakhin. Institute of Terrestrial Magnetism, ...
JOURNAL OF GEOPHYSICAL RESEARCH, VOL WI, NO. A12, PAGES 26,759-26,767, DECEMBER I, 1996
Study of the ionospheric variability within the Euro-Asian sector during the Sundial/Atlas 1 mission S. A. Pulinets and K. F. Yudakhin Institute of Terrestrial Magnetism, Ionosphere and Radiowave Propagation, Russian Academy of Science, Troitsk
D. Evans Space Environment Center NOAA, Boulder, Colorado
M. Lester Department of Physics and Astronomy, University of Leicester, Leicester, England
Abstract. In order to quantify, and to identify possible origins of, subauroral ionospheric variability during periods of moderate geomagnetic activity, ionospheric observations taken during the SUNDIAUATLAS - 1 campaign (March 24 to April 2) from 10 stations were analyzed in conjunction with observations from EISCAT, geomagnetic observations from magnetometer networks in Scandinavia and the United Kingdom, and auroral particle energy input observations from the NOAA - 12 satellite. The network of ionospheric stations spanned longitudes from 13~ to 90~ but were relatively confined in geomagnetic latitudes so that longitudinal and local time dependencies in ionospheric variability are more clearly exposed. The ionospheric observations were analyzed in terms of both llfJ'2, the difference between the hourlyfJ'2 at a given station and tlle mOJlthly medianfJ'2 for that hour, and a new daily variability index AfJ'2. The analysis using both parameters demonstrated an apparent longitudinal variation in ionospheric variability with a reversal at about 55° E from a negative to a positive phase in the departure of ionospheric conditions from their median values. An analysis of these ionospheric data in conjunction with the NOAAlrIROS estimates of pOwer deposition by auroral particles demonstrated a significant local time dependence in midlatitude ionospheric responses to auroral activity. This dependence may arise from the premidnight to postmidnight asymmetry in high-latitude convection electric fields.
1. Introduction The nature and ongm of day-to-day variability in the ionosphere is an important problem in modem ionospheric physics Gulyaeva et 01., (1990). The problem is usually set down in the framework of the "quiet" and the "disturbed" ionosphere which, in tum, requires a quantitative defmition of quiet and disturbed. The quiet ionosphere is generalIy viewed as one that exhibits a regular behavior welI described by daily, seasonal, and solar cycle variations. This regular behavior is termed "climatology" Szuszczewicz, (1995) and has been described by a variety of empirical or theoretical models Schunk and Szuszczewicz, (1988); Bilitsa et 01., (1992), although work continues to better defme the quiet ionosphere Gulyaeva et 01., (1995). The other extreme, the strongly disturbed ionosphere observed during isolated large geomagnetic storms, also appears to be theoreticalIy well described by first principle models Fuller-Rowell et 01., (1994, 1996). While the nature and origin of ionospheric variability both during quiet periods and associated with isolated, global Copyright 1996 by the American Geophysical Union. Paper number 96JA02411.
o148-0227196196JA-024 11$09.00
geomagnetic storms seems wel1 accounted for, the nature and origin of variability during periods (especially extended periods) of moderate geotnagnetic activity is much less well understood. There are a variety of reasons why it is difficult to account for ionospheric variability under these conditions. One reason is that conventional magnetic activity indices such as Kp and Ap are often unsatisfactory predictors of ionospheric variability as dempnstrated by situations where the level of ionospheric disturbance varies widely although the magnetic activity indices remain the same. A second reason is associated with the "chaotic" nature of long periods of moderate geomagnetic activity when a large number of substorms occur but at varying intervals, amplitudes, and locations with respect to an observing station. In this case the ionospheric variability observed at that location is the integrated result of numerous causes whose individual influences are difficult to separate. Still a third reason may be that the actual specification of ionospheric variability at a given station may not be as quantitative as desired which would introduce uncertainties in determining the Prigin of that variability. Conventional approaches to modeling ionospheric variability using statistical models of high latitude convection and auroral particle energy inputs typical1y give poor results during extended periods of moderate geomagnetic activity because of the difficulty in incorporating the time and spatial variations that are important during such conditions. Other studies have attempted to expose regular, periodic variations from
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PULINETS ET AL.: IONOSPHERIC VARIABILITY DURING SUNDJAUATLAS -1 MISSION
Table 1. Parameters of Ionospheric Database Used in Analysis
2 3 4 5 6 7 8 9
10
Station Name
Station Code
Longitude, 0 E
Juliusrch Kalinigrad
JUL KGD
13.3 20.62
54.6 54.7
Leningrad* Moscow Arkhangelsk Sverdlovskt Tashkent Karaganda Novosibirsk Podkamennaya Tunguska
LGD MSK ARK SVR TAS KRG NVS PKT
30.7 37.3 40.5 61.1 69.0 73.1 83.2 90.0
59.95 55.5 64.5 56.7 41.3 49.8 54.6 61.6
Latitude, 0 N
L
ILAT
UT
Data Availibility DataSet
~.6
2.6
51.2 51.4
IH IH
01.03-30.04 24.03-08.04
3.3 2.6 4.1 2.7 1.5 2.0 2.4 3.3
56.0 51.5 60.3 52.6 37.2 45.8 50.5 57.3
2H 2H 3H 4H 4H 5H 6H 6H
24.03-31.03 01.03-30.04 24.03-31.03 24.03-31.03 24.03-02.04 24.03-31.03 24.03-31.03 24.03-02.04
all
f0F2, M3000 all all
f0F2 all all all all all
* Now Sankt-Peterburg. t Now Ekaterinburg. ionospheric data in an effort to describe ionospheric behavior under these conditions (Lastovicka and Mlch, 1994a, b). Still other studies have attempted to develop empirical relationships between ionospheric variability and geomagnetic activity by taking advantage of the full range of magnetic activity indices (Kp,Ap, D1t, and Ae). Examples of this latter approach to model the trough position have been presented by Kohnlein and Raitt (1977) and Karpachev et al. (1995), while Kishcha (1995) used this approach to improve the empirical nu. The purpose of this paper is to develop an improved description of ionospheric variability under conditions of prolonged moderate geomagnetic activity. To this end, a correlative analysis was performed of observations obtained during March 1992 from a network of ionospheric stations, primarily in Russia but extending to Germany on the west, magnetometer observations from the United Kingdom and Scandinavia, European incoherent scatter (EISCAT) ionospheric observations, and auroml particle observations from the NOAAfI1ROS satellites. This period encompassed the SUNDJAUAlLAS Mission of March 24 through April 2 fully described by Szuszczewicz. et al. (this issue). The next section describes these observations in more detail, particularly the geophysical conditions and ionospheric variations during March 1992. Section 3 describes the ionospheric variability in terms of an index derived from those observations. Section 4 describes the NOAAfI1ROS auroral activity index and its association with midlatitude ionospheric variations. The last Arkhangelsk
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LONGITUDE (E) Figure 1. The geogmphic distribution of the European-Asian ionospheric stations used in this study.
section presents the conclusion that the use of indices that provide more physical characterizations of both ionospheric variability and geomagnetic activity improves the ability to separate the origins of ionospheric variability.
2. Observations During March 1991 This study utilizes observations, primarily of/J'2, obtained by a Russian network of nine ionospheric stations augmented at the western edge by a tenth station at Juliusruh, Germany. Figure I shows the geogmphic distribution of these station!; extending over the longitude range 130 to 900. Table I lists these stations with their invariant magnetic latitudes and the observational data that is available. Figure 1 shows that 7 of the 10 stations are in the geogmphic latitude range SOD to 600 N while Table I shows these same seven stations are in the invariant magnetic latitude range 500 to 57". The highestlatitude station, Arkhangelsk, is at a latitude which will be affected by the subauroral ionospheric trough. The lowestlatitude station, Tashkent, is well displaced from the auroml zone and during March 1992, displayed no variations associated with geomagnetic activity. The absence of highlatitude processes influencing the ionosphere above lowlatitude stations was also observed in the Austmlian-Japanese sector during the SUNDIAUAlLAS - 1 period Wilkinson et al., (this issue). Because of the clustering in latitude, this network is well placed to investigate longitudinal variations in the ionosphere. These ionospheric data were supplemented by magnetometer observations from the IMAGE and SAMNET magnetometer networks in the United Kingdom and Scandinavia and by EISCAT observations of the ionosphere over Scandinavia. These observations served to identify auroral and geomagnetic activity local to the European sector that might otherwise not be exposed by global geomagnetic activity indices such as Kp, Ap, and D .. that are also available for this study. Finally, use was made of the estimates of the power input to the polar regions in the form of auroml particle precipitation that are made from the NOANI1ROS satellites Foster et al., (1986). These estimates are made for each satellite tmnsit of the polar regions, which takes ahout 25 min. and are made twice per orbit or about every 50 min. The time required to obtain the data for each estimate, the time between estimates, and the fact that the estimate is partially based upon a statistical description of the auroral oval mean that these power
PULINETS ET AL.: IONOSPHERIC VARIABILITY DURING SUNDIAIJA1LAS - 1 MISSION input estimates do not expose either time variations of less than 1 hour nor localized spatial structure on the part of the power input to the auroral regions. The period March 1-31, 1992, was chosen for analysis with special attention to the SUNDIAIJATLAS - 1 campaign of March 24 to April 2 (although the analysis here does not extend beyond March 31). Figure 2 is a synopsis of geomagnetic and ionospheric activity during March 1992 as displayed by a variety of indices. Figure 2a displays both the Kp and Ap geomagnetic activity indices. Note that these indices are plotted with increasing activity downward to match the form of the D., index. The open circles in Figure 2b plot the AJrF2 ionospheric variability index derived from Moscow station observations during March 1992. The AJrF2 index has been proposed by Gulyaeva et 01. (1990) as providing a quantitative parameterization of ionospheric variability and has the advantage of being directly derived from ionospheric observations without reference to any other index. AJrF2 is dermed as one - half the difference between the absolute maximum and absolute minimum values of JrF2 observed by the station during a given day.
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5
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Figure 2. (a) The 3-hour Kp and daily Ap geomagnetic activity indices during March, 1992. (b) Open circles: the daily AfoF2 index of ionospheric variability during March, 1992 for the Moscow observing station. Solid circles: the daily averages of the NOAAlI1ROS estimates of auroral particle power input during the SUNDIAIJA1LAS - 1 campaign, plotted with increasing power downward. (c) The pass-by-pass estimates of auroral particle power input during Ule SUNDIAIJATLAS - 1 period (plotted with increasing power downward) apd the hourly D., index for March 1992.
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(1) Also plotted as solid circles in Figure 2b are the daily means of the estimated power input derived from the NOAAlI1ROS observations. The units are gigawatts and plotted with increasing power input downward. Figure 2c displays the I-hour D., index for March 1992 and the pass-by~pass (roughly I-hour resolution) estimates of power input. These power estimates are plotted in units of gigawatts and are also plotted with increasing power downward. The solid bar in Figure 2a-2c highlights the SUNDIAIJATLAS - 1 campaign period. Figure 3 is a more detailed synopsis of ionospheric variability during March 1992 as observed by the ionospheric stations at Moscow and Iuliusruh. The top section in Figures 3a and 3b shows, first, the median diurnal variation inJrF2 for this month (solid curve) and, second, the actual observed hourly JoF2 values (triangles). The bottom section in each panel displays the difference of the hourly JrF2 from the median monthly value in the form of a percentage. The bottom section illustrates the day-to-day and hour-by-hour variability of the ionosphere over each station during March 1992. Figures 2 and 3 provide an overall characterization of geolI}agnetic and ionospberic activity during this month. It is clear that the !mtire month is moderately disturbed. There were only ~ days when the Kp index remained below 3- for the entire day (March 14, 19, and 20). At the same time there were no instances when Kp exceeded 5+, and the highest Ap was 26 on M!lrch 23. March 1992 contained no major magnetic storms but was dominated by lengthy periods of moderate geomagnetic activity, notably March 1-11 when Ap dropped below 10 only once and March 21-31 when Ap did not drop below 10 at all. This latter period of prolonged activity coincided with the SUNDIAIJATLAS - 1 period. Observations from the European magnetometer networks also show that numerous substorms occurred during the SUNDIAIJATLAS - I period, often twice a day near 1930 and 0000 {IT (see Section 3). One expects that iOl}ospheric variations will be influenced both by global activity, as parameterized by Kp or Ap, and local substorm activity. The attentpt to separate the two is an objective of this analysis. The D., indices during March 1992 (Figure 2c) confirm this assessment. The hourly D., rarely was larger than -20 nT during the month and during three periods (around March, 1, 17, and 21) decreased below the -50-nT level that Gonzales et 01. (1994) would derme as moderate storms. The fust of these intervals was, in fact, the recovery phase of a storm that began on February 29 when D.., reached a minimum of -133 nT on that day. The AJ0F2 index Gulyaeva et 01. (1990) for Moscow for March 1992 is presented in Figure 2b (open circles). The AJrF2 index reflects very well the main features of D" (Figure 2c) especially the minor storm events on March. 16 and 21 The calculated correlation between AJrF2 and D., for March was of order 0.7. Comparisons ofAJrF2 withAp and Kp, however, do not give such good correlation with coefficients of 0.3 and 0.2, respectively. A detailed discussion of the physical meaning and differences between geomagnetic indices can be found in the work by Gonzales et 01., (1994). and more information on the correlation between the different indices, with better statistics, by Saba et 01. (1994). The comparison described above suggests a physical cause of the variation in the derived AJrF2 index for the March interval. D., reflects the strength of Ule ring current and the apparent strong correlation with AJoF2 suggests an influence on the middle latitude and subauroral
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PULINETS ET AL.: IONOSPHERIC VARlABll.JTY DURING SUNDIAUATLAS - I MISSION
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Juliusruh March 1992 Figure 3. Synoptic ionospheric observations from the Moscow and luliusruh stations for March, 1992. The top portion of each panel displays the median hourly values of/J'2 for March (solid curve) and the actual observed hourly value of /J'2 (triangles). The bottom portion of each panel displays !:ifJ'2, the hourly percentage deviation of the observed/J'2 from its monthly median value.
ionosphere during the magnetic storm development. The lack of correlation between Ap and Kp with A/oF2 indicates therefore that the general ionospheric currents which cause the variations in these indices do not play a key role in the ionospheric variability. The ring current effect which is hinted at therefore consists of a suppression of the daily variations of the critical frequency. Naturally, this conclusion should be supported by a larger statistical study for a number of ionospheric stations as well as different levels of solar and geomagnetic activity. The/oF2 observations presented in Figure 3 also show that March 1992 was characterized by moderate activity nearly throughout the month. Both negative and positive departures of /oF2 by more than 30% from the median hourly values are observed on nearly every day. The Moscow data in particular illustrates the poor association of ionospheric variability with
global magnetic indices. Departures of -30% from the median on March 5, 6, and 18 are seen in these data. Yet while ionospheric variability was the same, the Ap indices for these 3 days were 12, 8, and 17, respectively. There are also differences between Moscow and luliusrull observations in Figure 3, which demonstrate longitudinal variability of the ionospheric density. The luliusrull data seem to be modulated by magnetic activity to a greater extent than Moscow. Significant minima in !:ifJ'2 at luliusruh occur on March 1,9, 11, 18, and 23 which are all related to mininla in DII and maxima in Ap. These minima in !:ifoF2 at luliusruh, apart from the one on March 9, are followed by recovery to higher densities over the Qext few days. This type of behavior does not exist at Moscow during this interval. A second feature which demonstrates longitudinal variability occurs during the
PULINETS ET AL.: IONOSPHERIC VARIABILITY DURING SUNDIAUATLAS - I MISSION Date
:15 10
Longitude, deg (E) Figure 4. Dynamics of 4fJil (in percent) variations for SUNDIAUA1LAS - I in the Euro-Asian sector. The magnitude of 4fJil is presented in a contour gray scale fonnat. Time increases from top to bottom. Periods of substonn activity are shown by bold line along the left vertical axis. The bold black arrow indicates the precipitation event registered at Arkhangelsk station. SUNDIAL interval at the end of March. Here, although both stations reflect negative phase stoon behavior, Moscow typically has larger and longer lived negative values of 4foF2. The longitudinal differences in ionospheric variability during periods of moderate magnetic activity, apparent in comparing the Juliusruh and Moscow observations may very well reflect the spatial structure and time variations in auroral particle precipitation and ionospheric convection during substonns. Further investigations of such longitudinal variations are discussed in the next section.
3. Longitudinal Variations in Ionospheric Density To investigate longitudinal variations in ionospheric density, we have analyzed vertical sounding data collected from ten ionospheric stations situated within the longitudinal range 13° to 90° E. As mentioned above the distribution of the stations is such that we can concentrate on longitudinal variations alone. We note, however, that the high-latitude station may be significantly different from the other stations, because of its general close proximity to the main F - region trough. To investigate the deviation in/oF2, we used hourly values of the critical frequency for the period of March 24 through 31 rather than the monthly medians. The percentage difference in/oF2, is plotted as a function of longitude and time in Figure 4. Longitude is shown along the X - axis and time along the Y axis from top to bottom, with the deviations in critical frequency expressed by gray scale fonnat and zero deviation of critical frequency marked by solid black lines. The time of local midnight is shown by the bold white lines. To produce contours of ionospheric variations for ionospheric stations which are not equally spaced in longitude, Kriging technique was used Oliver and Webster (1990). Kriging uses regional variables theory Matheron (1971) to
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calculate the autocorrelation between data points and is based on weighted averages, where weights depend on the distance separation as given from the semivariogram. The semivariogram characterizes the separation distance dependence of the semivariance, V, which is the sum-squared difference between values. For small separations data values are similar and V is loW; for large distances values are uncorrelated and V attains a limiting 'sill' figure equal to the variance. The same technique was used also to build the distributions in Figure 6 and Figure 7. The ionosphere during the SUNDIAUA1LAS - I interval is highly variable in this longitude sector. The deviation in/oF2 at most of stations reaches, and sometimes exceeds, :i: 40%. There are several positive extremes, with the most marked occurring in the evening of March 27 at the western end of the station chain. Such large positive variations may be attributed to local strong precipitation events. One such event on March 27, observed by the Arkhangelsk station, is marked by the bold black arrow in the Figure 4. EISCAT, which is located 30° west and 5° poleward of Arkhangelsk, ran a mode similar to CP-l for 6 hours on March 27 Lester et al. (this issue). In this mode the UHF transmitter at Tromso is pointed in a direction which is parallel to the magnetic field in the F - region above Tromso. The beam directions of the two remote sites intersect the transmit beam at 275 km altitude. Electron density and temperature measurements between ISO and 600 km altitude for the interval 1600 to 2200 Uf are plotted in the top two panels of Figure 5. The percentage deviation in /oF] for the whole of March 27 is given in the third panel of Figure 5 and the H component of the Kilpisjarvi magnetogram for the whole day in the lower panel. A substonn is clearly identified in the magnetogram at about 2000 Uf, and this is discussed in more detail by Lester et al. (this issue). Substonn activity had also been present earlier and particle precipitation into the E and F regions was present between 1830 and 1900 Uf. The large peak in the deviation in/oF2 at Arkhangelsk occurs at the time of the substonn activity identified to the west by EISCAT. Furthermore, a pronounced peak in deviation from/oF2 is also observed at the next highest-latitude station in the western part of the station coverage, Sank! Petersburg at the same time. The effects of energetic electron precipitation upon the auroral ionosphere has been studied by many authors, but the most systematic approach can be found in the work of Mishin et al. (1989, 1990). Accelerated electrons precipitate into the upper ionosphere and, depending upon the initial energy spectrum, cause many effects such as raising the electron temperature within the upper ionosphere, increased ionization in both F and E - layers, and the formation of the thin layers of enhanced ionization in the height range between 100 and 120 km. The increased F - region ionization measured by EISCAT during this interval is consistent with the enhanced critical frequencies at Arkhangelsk and Sankt Petersburg. The Kilpisjarvi magnetometer data illustrated the localized The IMAGE and isolated substonn on March 27. magnetometer array Lester et al. (this issue) can provide evidence of substonn activity in the Scandinavian sector from about 1700 to 0600 Uf. Investigation of the data reveals that substonns occurred in the premidnight local time interval on each of the nights from March 24 through 31 inclusive, and these intervals are marked on Figure 4 by the solid bar along the time axis. On occasion these were isolated substorms, as in the case of March 27, and on other days the activity was more continuous, for example, from 1845 Uf on March 30 to 0300
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