RECENT RESULTS FROM CORRELATIVE IONOSPHERE AND
MAGNETOSPHERE STUDIES INCORPORATING
Rosenberg and the AGO Science
Institute for Physical Science
University of Maryland, College Park, Maryland, 20742-2431, USA
New facilities established in the southern and northern polar regions
in recent years have enabled extensive and simultaneous coverage of both
polar ionospheres. The distributed high latitude ground-based observations
are now being interpreted within the context of in-situ data being provided
by a variety of ISTP-era
spacecraft. This has created new opportunities for global (including conjugate)
studies of particle precipitation, magnetic pulsations, ionospheric currents,
radiowave emissions, and the structure and dynamics of the ionospheric
plasma. This paper highlights several recent studies emphasizing the results
of measurements made in Antarctica.
Extensive and simultaneous observation of the southern and northern polar ionospheres is now possible with the many ground-based facilities that have been established. These include the U.S. and British arrays of Automatic Geophysical Observatories (AGOs) in Antarctica, the SuperDARN HF radars in both hemispheres, and the northern chains of magnetometers and other instruments located in Canada (CANOPUS, MACCS), Greenland, and Scandinavia (IMAGE). Many opportunities exist for coordinated ground-and satellite-based observations with the ISTP-era spacecraft currently available. Such studies can lead to the calibration of ground signatures of magnetospheric-ionospheric processes which will provide for an ongoing remote sensing capability.
Several research topics have been selected for this presentation. These include 1) substorm auroral expansions to very high latitudes; 2) a southern high-latitude geomagnetic index; 3) localization of an ionospheric perturbation accompanying a magnetic impulse/traveling convection vortex event; and 4) Pi 1 magnetic pulsations in space and at high latitudes on the ground. These topics focus on the contributions of Antarctic measurements, primarily arising from the U.S. program of manned and automatic observatories (Engebretson et al., 1997; Rosenberg and Doolittle, 1994). Only the main results will be summarized here; additional details can be obtained from the references cited.
The U.S. AGO network consists of a suite of nearly identical instruments placed at six specifically-sited locations on the Antarctic polar plateau. The instruments, the responsible investigators, and their institutions are listed in Table 1. The U.S. AGOs are deployed at the sites labeled AP1-AP6 in Figure 1a (squares). For reference, the locations of British AGOs (triangles), which complement the U.S. program at lower geomagnetic latitudes, are also given in Figure 1a, as are the locations of several key manned stations. For additional reference, Figure 1b also depicts the conjugate projections onto Greenland and Canada of the few Antarctic sites that form close conjugate pairs with northern hemisphere locations (e.g., SPA/IQA, AP3/STF, AP1/CLY (a MACCS station)). Table 2 gives the coordinates of all sites identified in Figure 1a and 1b.
a) b) (Click figures for larger versions!)
Figure 1: Antarctic (a) and Arctic (b) observation sites (see
Table 2 for identifications). Geographic (geomagnetic)
coordinates are given by the dotted (solid) lines. Several nominally
conjugate station pairs are depicted on the Arctic map.
The U.S. AGOs form two geomagnetic meridional arrays. One (AP2, SPA,
AP1, AP5, AP6) is along the meridian that includes South Pole station and
stretches from the latitude of the polar cusp (approximately 70 degrees
geomagnetic latitude under highly disturbed conditions) through the pole
of the dipole magnetic field between AP5 and AP6. The second array (AP3,
AP4, AP5, AP6) is situated about 1.6 hours earlier in magnetic local time.
The AGOs at AP1 and AP4, together with the manned stations McMurdo, Casey,
Dumont d'Urville, and Terra Nova Bay, provide a longitudinally spaced array
at 80 degrees geomagnetic latitude to give coverage in the polar cap over
an extended range of magnetic local time. This cannot be achieved from
land-based sites in the northern hemisphere.
SELECTED RESEARCH TOPICS
Substorm Auroral Expansions to Very High Latitudes
Although spacecraft have observed a variety of polar cap auroras, there has been relatively little discussion in the literature about the detailed spatial and temporal morphology of extremely high latitude substorm disturbances, apart from the reports by Gussenhoven (1982) and Craven and Frank (1991). The maximum poleward extent of substorm precipitation effects can easily reach a magnetic latitude of 75 degrees, but it is much less common to observe effects extending beyond 80 degrees.
Weatherwax et al. (1997) used riometer data from AP1, AP4, and MCM to track the morphology and progression of auroral absorption features over 5 hours of local time at 80 degrees magnetic latitude. Two examples are shown in Figure 2, corresponding to cases when the stations were pre-midnight (left panels) and post-midnight (right panels). The azimuthal motions observed, westward pre-midnight, eastward post-midnight, are similar to the dynamics expected closer to the auroral zone. Weatherwax et al. (1997) found that substorm-related absorption events are observed deep into the polar cap in the late expansion/recovery phase of a substorm during moderate-to-strong planetary disturbances and at times of high solar wind speed conditions (a relationship also noted by Sergeyev et al., 1979). ULF modulation of the absorption (and hence of the causative fluxes of precipitating energetic electrons) could also occur on occasion. These modulations were usually dominated by spectral peaks of 1-4 mHz and were confined to a narrow, 1 latitude band embedded within more uniform precipitation (see Weatherwax et al. 1997).
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Figure 2: Equivalent overhead/broadbeam riometer absorption
MCM, AP1, and AP4, all at 80 magnetic latitude. The magnetic local time clock shows that the stations were in the evening-to-midnight sector at 2300 UT on May 29, 1994 (left panels) and were in the midnight-to-dawn sector at 0730 UT on May 10, 1994 (right panels).
(MMLT = midnight magnetic local time)
More recently, Doolittle et al. (1998) examined several cases of substorm expansion with all-sky cameras at the (available) AGO sites. Figure 3 is a mosaic of auroral images when the observing sites were near midnight and shows the aurora expanding into the polar cap beyond 83 degrees. Other examples, when the sites were near dusk or near dawn, showed the aurora advancing westward or eastward, respectively, as it was expanding poleward, consistent with the observations of Weatherwax et al. (1997). Summarizing, in the pre-midnight (post-midnight) sector, substorm expansion into the polar cap appears as a westward (eastward) motion progressing towards the dusk (dawn) flank. The optical and absorption observations are consistent with an oval that expands poleward with greater time delay the further the site is from local midnight. During recovery, the equatorward motion of the aurora occurring first near magnetic midnight continues the apparent motion of the aurora towards the dawn and dusk flanks. All cases considered to date have been associated with high solar wind speeds (> 600 km/s) and external triggering (northward-turning from Bz south).
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Figure 3: 630.0 nm all-sky images mapped to MLT vs. magnetic longitude on 08 May 1995 at 0251 UT.
Southern High-Latitude Geomagnetic Index
Data from the set of fluxgate magnetometers that are included in the
AGO array at 80 and higher southern latitudes (AP1, AP4, AP5, AP6), together
with data from MCM, DRV, and CSY, are being used to investigate possible
polar cap geomagnetic indices. The development of such an index, including
understanding its features in the context of magnetospheric dynamics, could
contribute importantly to studies of physical processes in the polar cap.
The present day Polar Cap Index uses data from a single station in each
polar hemisphere (Thule in the north and Vostok in the south) to derive
a measure of the polar cap current systems there. One investigation in
progress is to determine the universal validity of this index, in both
time and in longitude at a specific UT, given recent results from studies
of AGO data (see above) that demonstrate that magnetospheric substorms
can surge poleward as far as AP1 and AP4 (and Vostok, which is at a considerably
lower geomagnetic latitude than is Thule).
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Figure 4: Correlations vs. time of geomagnetic indices with Kp, Dst, and IMF-Bz component. The top panels show correlation coefficients for AU() and AUS-80(); the middle panels for AL ()and ALS-80 (); the bottom panels for AE () and AES-80 (). Each point is shown at the center of the 2-hour interval to which it refers.
Also under investigation is an index (AES-80) that is derived in analogy to the northern hemisphere auroral electrojet (AE) index (Ballatore et al., 1998a,b,c). Previous attempts to determine a large-scale southern hemisphere activity index similar to AE (Maclennan et al., 1991) were constrained to the use of non-optimally-spaced station distributions (the present manned stations). Ballatore and colleagues used data from the five 80 degree sites at AP1, AP4, MCM, DRV,and CSY for the two months of May-June 1994. They computed AUS-80, ALS-80, and AES-80 from 1-minute H-component values, after subtracting the average of the two quietest days of the month. A comparison of the behavior of AE and AES-80 with the planetary index Kp, the ring current index Dst, and the north-south component of the interplanetary magnetic field Bz, is shown in Figure 4. The best correlation is found with the Kp index, indicating that both AE and AES-80 are in good agreement with activity on a planetary level. In addition, there is good agreement between AE and AES-80 independent of time (bottom panels). The agreement is less satisfactory for the AL and AU components, particularly for the correlation of Kp with AUS-80 (top left panel) between about 06 and 22 UT. This difference, which may possibly be related to seasonal dependence or to station distribution, requires further study.
Localization of an Ionospheric Perturbation Accompanying a Traveling Convection Vortex Event
Sitar et al. (1998) analyzed a sequence of traveling convection vortex (TCV) events observed over Greenland/Eastern Canada from 1030-1200 UT on July 24, 1996. The TCVs were consistent with alternating pairs of upward and downward field aligned currents (FACs) moving tailward in the morning sector at 10 km/s. The main current flow was at 77- 79 degrees geomagnetic latitude. A localized intensification of auroral emissions in this latitude range accompanied the trailing vortex (upward FAC) of the last and most intense TCV at 1140 UT. The auroral brightening and TCV enhancement have been attributed by Sitar et al. (1998) to the arrival at Earth of a sudden change of IMF orientation, leading to the formation of a hot flow anomaly on the bow shock. This was observed as two rapid transits by the Interball 1 spacecraft, within a time span of 7 minutes, from the solar wind to the magnetosphere and back to the solar wind (Sibeck et al., 1998).
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Figure 5: Magnetometer data from SPA and AP3 and riometer and photometer data from SPA for the magnetic impulse/TCV event of July 24, 1996.
Weatherwax et al. (1998) have considered conjugate aspects of this event using data from the two conjugate pairs, SPA/IQA and AP3/STF (see Figure 1b). Magnetometer data from SPA and AP3 for this event are shown in Figure 5 along with riometer and photometer data from SPA. Two large (>100 nT) negative H-component magnetic impulses, spaced 8 min apart, are evident at SPA. Two smaller magnetic impulses occurred 3 min earlier at AP3, consistent with tailward propagation of the disturbance (as was found for the northern hemisphere), but at the slower rate of 3 km/s. Precipitation of energetic electrons, as manifested by impulsive increases of riometer absorption and 427.8 nm brightness, occurred with the second of the SPA magnetic impulses during a period of downward FAC flow.
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Figure 6: Meridional (top panel) and azimuthal (bottom panel) riograms of the ionospheric absorption at SPA accompanying the magnetic impulse event at 1143 UT on July 24, 1996. The south-north (west-east) riogram is constructed from the row (column) average of beams in the three central columns (rows) of the 49-beam array. The ordinate scale gives the horizontal distance from overhead at SPA assuming that the main absorbing height is at 90 km.
Additional details of the riometer absorption at SPA between 1140 and 1145 UT are shown in the meridional (top panel) and azimuthal (bottom panel) riograms of Figure 6. Absorption began near 1141 UT and ended by 1144 UT. The perturbed region remained mostly equatorward of SPA until near the end when it moved slightly poleward of the station. The motion was more varied in the azimuthal direction, initially directed to the west (tailward), then to the east, and finally back to the west, suggesting that the perturbed region remained in close proximity to the station. No corresponding riometer absorption was evident at AP3 or at either of the two northern conjugate stations IQA and STF at this time, further indicating that the precipitation was quite localized and/or that nominally conjugate field lines may have been significantly distorted. There is also evidence from the 630.0 nm emission at SPA (see third panel in Figure 5) that much of the initial auroral current was caused by the precipitation of low energy electrons following the magnetic impulse at 1133 UT.
Pi1 Magnetic Pulsations in Space and at High Latitudes on the Ground
Pi1 are irregular magnetic pulsations in the period range of 1-40 s which can appear as bursty broadband signals (Pi1B) or as continuous narrowband signals (PiC). They have long been associated with auroral electron precipitation and are generally believed to be the ground signature of overhead ionospheric currents created by enhanced conductivity due to the precipitation. Arnoldy et al. (1998) have just completed a high latitude (68-80 degrees geomagnetic) investigation of Pi1 waves using AP1, AP2, AP3, AP4, the British AGO A81, and induction magnetometers at SPA, STF, and IQA (see Figure 1). Their objective was to determine the time development of the ground Pi1B in the region where auroras rapidly move poleward during the expansive phase of substorms. Line plots and spectrograms of the ground pulsation data for one example are shown in Figure 7. An important component of the investigation was to look at simultaneous GOES magnetometer data from the equatorial plane within a few tens of degrees longitude of the ground sites (see Figure 2 of Arnoldy et al., 1998).
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Figure 7: Line plots and spectrograms of pulsation data from sites identified in left column (here SS and SP refer to Sondrestrom and South Pole, respectively) covering magnetic latitudes from 70-80.
From these and other examples discussed, Arnoldy et al. (1998) conclude that 1) Pi1B waves are seen in space associated with magnetic field dipolarizations on the nightside while PiC waves (in their study) are not seen in space; 2) the lowest latitude ground station recording Pi1B detects these waves riding on a strong and sudden westward electrojet signal which is not evident at the higher latitude stations; 3) the higher latitude stations see a maximum Pi1B signal delayed consistent with auroral motion poleward; 4) there is a prompt Pi1B signal seen (without delay) at other latitudes and longitudes, whose onset time is that of the signal recorded at the lowest latitude station; 5) PiC across the entire array seems to be initiated by this same onset Pi1B signal; and 6) ground Pi1 waves, particularly morning PiC, are closely associated with overhead particle precipitation for frequencies below 0.1 Hz.
These observations suggest that Pi1B waves are generated in space and propagate along field lines that carry the tail reconnection current. The upward currents at the head of the auroral surge are the auroral particles that enhance the ionospheric conductivity and create overhead currents that significantly contribute to the Pi1B. It is suggested that the magnetospheric Pi1B waves could excite a resonance cavity creating the PiC waves that are seen over the entire array. This same cavity could also horizontally duct the Pi1B waves leading to the prompt Pi1B signal over the array.
The U.S. AGO program in Antarctica is sponsored by the National Science Foundation through grant OPP-9529177 to the University of Maryland. Additional grant support of research described here also came from the National Science Foundation as follows: OPP-9505823 (University of Maryland), OPP-9613683 and OPP-9316750 (University of New Hampshire and Augsburg College). The U. of New Hampshire work was also partially supported by NASA under grant NAG5-5007. The author especially wishes to thank the AGO Science Team (R.L. Arnoldy, C.G. Maclennan, J.H. Doolittle, M.J. Engebretson, H. Frey, J. LaBelle, L.J. Lanzerotti, S.B. Mende, and A.T. Weatherwax) for their contributions to this report.
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