Alicia L. Clua de Gonzalez

Interplanetary Origin of Geomagnetic Activity in the Declining Phase of the Solar Cycle

Interplanetary magnetic field (IMF) and plasma data are compared with ground-based geomagnetic Dst and AE indices to determine the causes of magnetic storms, substorms, and quiet during the descending phase of the solar cycle. In this paper we focus primarily on 1974 when the AE index is anomalously high . This year is characterized by the presence of two long-lasting corotating streams associated with coronal holes. The corotating streams interact with the upstream low-velocity (300–350 km s−1), high-density heliospheric current sheet (HCS) plasma sheet, which leads to field compression and ∼ 15- to 25-nT hourly average values. Although the Bz component in this corotating interaction region (CIR) is often −25 nT). Storms of major (Dst ≤ −100 nT) intensities were not associated with CIRs. Solar wind energy is transferred to the magnetosphere via magnetic reconnection during the weak and moderate storms. Because the Bz component in the interaction region is typically highly fluctuating, the corresponding storm main phase profile is highly irregular. Reverse shocks are sometimes present at the sunward edge of the CIR. Because these events cause sharp decreases in field magnitude, they can be responsible for storm recovery phase onsets. The initial phases of these corotating stream-related storms are caused by the increased ram pressure associated with the HCS plasma sheet and the further density enhancement from the stream-stream compression. Although the solar wind speed is generally low in this region of space, the densities can be well over an order of magnitude higher than the average value, leading to significant positive Dst values. Since there are typically no forward shocks at 1 AU associated with the stream-stream interactions, the initial phases have gradual onsets. The most dramatic geomagnetic response to the corotating streams are chains of consecutive substorms caused by the southward components of large-amplitude Alfven waves within the body of the corotating streams. This auroral activity has been previously named high-intensity long-duration continuous AE activity (HILDCAAs). The substorm activity is generally most intense near the peak speed of the stream where the Alfven wave amplitudes are greatest, and it decreases with decreasing wave amplitudes and stream speed. Each of the 27-day recurring HILDCAA events can last 10 days or more, and the presence of two events per solar rotation is the cause of the exceptionally high AE average for 1974 (higher than 1979). HILDCAAs often occur during the recovery phase of magnetic storms, and the fresh (and sporadic) injection of substorm energy leads to unusually long storm recovery phases as noted in Dst. In the far trailing edge of the corotating stream, the IMF amplitudes become low, <3 nT, and there is an absence of large-amplitude fluctuations (Alfven waves). This is related to and causes geomagnetic quiet. There were three major (Dst ≤ −100 nT) storms that occurred in 1974. Each was caused by a nonrecurring moderate speed stream led by a fast forward shock. The mechanisms for generating the intense interplanetary Bs which were responsible for the subsequent intense magnetic storms was shock compression of preexisting southwardly directed Bz (Bs) for the two largest events and a magnetic cloud for the third (weakest) event. Each of the three streams occurred near a HCS crossing with no obvious solar optical or X ray signatures. It is speculated that these events may be associated with flux openings associated with coronal hole expansions. In conclusion, we present a model of geomagnetic activity during the descending phase of the solar cycle. It incorporates storm initial phases, main phases, HILDCAAs, and geomagnetic quiet. It uses some of the recent Ulysses results. We feel that this model is sufficiently developed that it may be used for predictions, and we encourage testing during the current phase of the solar cycle.

Periodic variation in the geomagnetic activity: A study based on the Ap index

The monthly and daily samples of the Ap geomagnetic index for 51 years, 1932-1982, were investigated by means of the power spectrum technique. In general, the results confirm previous findings about possible periodicities in the geomagnetic activity. However, in our opinion the following aspects are either new or they are being interpreted somewhat differently than other authors have done. The period around 4 years in the monthly Ap power spectrum is associated to the double peak structure observed in the geomagnetic activity variation [Gonzalez et al., 1990]. Several of the peaks shown by the daily Ap spectrum are interpreted as harmonics of the 6-month period and other peaks as caused by the solar rotation periodicity, in such a way that the two series of Fourier sequences are consider to be juxtaposed. A strong solar cycle modulation is observed in these series, particularly in that related to the solar rotation period, which almost disappears for the solar maximum phase. The study of the seasonal variation was complemented by a superposed epoch analysis. The profiles resulting from this analysis seem to show a multiple origin of the 6-month periodicity, so that it does not seem realistic to search for a unique cause for this well-known seasonal variation. This conclusion is also supported by the histograms of the occurrence of storms above a given intensity level, taken over short duration intervals (i.e., 8 days). According to these histograms, for large data samples the dates with largest number of storms are spread out around those predicted by the different theoretical models, while for short intervals the semiannual periodicity may sometimes not even be present. Therefore these known mechanisms would combine to give a resulting modulation of the geomagnetic response to the randomly generated source of storms. It was also found that an additional seasonal peak seems to exist in July, with an amplitude comparable to those of the equinoctial peaks, for the range of the most intense storms (Ap ≥ 150 nT). A weak periodicity around 158 days, well correlated to that of about 155 days observed in the solar activity, has also been detected for some years during solar cycle 21.

Dual-peak solar cycle distribution of intense geomagnetic storms

Abstract The solar cycle distribution of intense geomagnetic storms has been studied for cycles 20 and 21 (1965–1985), using values of the Dst index 100 nT. It is claimed that a dominant dual-peak (DP) distribution exists in the solar cycle variability of these storms, with one peak occurring at the late ascending phase of the cycle or at solar maximum and another at the early descending phase of the cycle. From the 10 cycles studied, the average separation of the peaks from solar maximum is about 8 months ahead for the first peak and about 25 months after for the second one. For the cycles studied, the ratio of the average number of storms occurring at the peaks as compared with that occurring at the valley of the DP distribution was found to be statistically significant to better than a 95% confidence level. Although the present study is restricted to intense storms, their distribution including moderate events for the interval 1965–1985, in terms of Dst values 3 h) values of the negative Bz component of the interplanetary magnetic field (IMF), computed for the interval 1970–1981. This class of Bz fields was shown to be the cause of all intense storms studied for the interval August 1978/2-December 1979 (Gonzalez and Tsurutani, 1987, Planet. Space Sci. 35, 1101).

The South Atlantic Magnetic Anomaly : three decades of research

Abstract This brief review of advances in our understanding of some physical processes related to the South Atlantic Magnetic Anomaly (SAMA) is intended to highlight specific issues on which further research is needed. The discussion focuses on the origin of the SAMA, the geomagnetic storm-related effects and the impact of the SAMA on orbiting spacecraft.

Annual variation of geomagnetic activity

Abstract The annual variation of geomagnetic activity is studied through the geomagnetic indices aa, Dst and AE, according to different levels of intensity for each of the indices. For thresholds that correspond from moderate to fairly intense storms (i.e., peak aa⩾ 90), the distribution follows the well-known pattern of a seasonal variation, with maxima around the equinoxes and minima near the solstices. Deviations from this behavior are observed when the distribution refers to levels associated with the occurrence of more intense storms. In particular, the annual distribution of days with a geomagnetic index aa greater than about 90, shows the occurrence of a peak in July. The contribution of very intense storms (aa⩾ 210) to the July peak, seems to be evenly distributed along the 12 solar cycles covered by this index. Furthermore, the indices Dst and AE, although restricted to a much shorter interval of time (they have been recorded only since 1957), seem as well to show the existence of a peak of occurrence for July. The study done for the indices Dst and AE gives some indication for the existence of another peak in November, also for thresholds associated with intense storms. However, due to the lack of longer records for these indices, the real existence of this peak in the geomagnetic activity is questionable. A statistical analysis of the distribution of events according to the levels of intensity of the aa and Dst is also presented. From this analysis it is seen that the number of occurrences of storms above a given level of intensity of those geomagnetic indices, can be approximated by an exponential law. Furthermore, an estimation of the occurrence of storms during a solar cycle as a function of the peak aa (or aa ∗ ) has been also done.

Interplanetary shock parameters during solar activity maximum (2000) and minimum (1995-1996)

Interplanetary shock parameters are analyzed for solar maximum (year 2000) and solar minimum (years 1995-1996) activity. Fast forward shocks are the most usual type of shock observed in the interplanetary medium near Earths orbit, and they are 88% of the identified shocks in 2000 and 60% in 1995-1996. Average plasma and magnetic field parameters for upstream and downstream sides of the shocks were calculated, and the parameter variations through the shock were determined. Applications of the Rankine-Hugoniot equations were made, obtaining shock speeds and Alfvenic Mach number. Static and dynamic pressures variations through the shocks were also calculated. Every parameter have larger variation through the shock in solar maximum than in solar minimum, with exception of the proton density. The intensity of shocks relative to the interplanetary medium, quantified by the Alfvenic Mach Number, is observed to be similar in solar maximum and minimum. It could be explained because, during solar maximum, in despite of the higher shock speeds, the Alfvenic speed of the interplanetary medium is higher than in solar minimum.

On the Solar Origins of Intense Geomagnetic Storms Observed During 6–11 March 1993

Intense geomagnetic storms with DST index ≤-100 nT were recorded on 9 March and 11 March 1993 associated with solar activity on 6 March and 9-10 March, respectively. In this paper, we discuss the characteristic features of the solar origins of the two events that gave rise to coronal and interplanetary disturbances and as a consequence produced strong geomagnetic activity at the Earth. The source of the activity in one case is attributed to a major 3M7.0 flare that occurred on 6 March 1993 and in the other case, to two large filament disruptions on the disk during 9-10 March, 1993. Both these sources were found to be located near changing or varying low-latitude coronal holes. They were also located close to the heliospheric currents sheets. Distinct X-ray activity was observed for both the events as observed by the Yohkoh SXT telescope. The detailed evolution and a comparison of these events on the basis of Yohkoh soft X-ray observations are presented here.

About the origin of peaks in the spectrum of inner belt electrons

A power spectral analysis of geomagnetic data between June and November 1982 from Vassouras, a low-latitude observatory located in the South Atlantic Magnetic Anomaly (SAMA), is carried out in order to investigate the origin of multiple peaks commonly observed in the energetic electron spectra in the inner magnetosphere. Comparison with energetic electron observations made by the low-altitude satellite S81-1 during the same time interval [Datlowe et al., 1985] indicates that even if the peaks are due to a quasi-resonance process as Cladis [1966] has suggested, whereby energetic electrons are accelerated and transported radially as a result of magnetic fluctuations with periods comparable to their azimuthal drift periods, the driver of such magnetic fluctuations is not the ionospheric current in the equatorial electrojet, modulated by electron precipitation in the SAMA region, such as Cladis [1966] proposed. We suggest that the source of drift-resonant magnetic fluctuations, if they exist, must instead be sought in the solar wind. Otherwise, the drift-resonant mechanism cannot be viable.

Role of the lifetime of ring current particles on the solar wind-magnetosphere power transfer during the intense geomagnetic storm of 28 August 1978

Abstract For the intense magnetic storm of 28 August 1978 it is shown that the power transfer from the solar wind to the magnetosphere is well represented by the expression obtained by Vasyliunas et al . (1982, Planet. Space Sci . 30 , 359) from dimensional analysis, but this representation becomes improved when such an expression is modified by a factor due to an influence of the lifetime of ring current particles as suggested by Lee and Akasofu (1984, Planet. Space Sci . 32 , 1423). During a steady state regime of the ring current evolution of this storm, our study suggests that the power transfer depends on the solar wind density, the transverse component of the IMF (with respect to the Sun-Earth line) and also, explicitly, on the time consant for ring current energy decay, but not on the solar wind speed.

On the equivalence of the solar wind coupling parameter ε and the magnetospheric energy output parameter UT during intense geomagnetic storms

Abstract For intervals with intense geomagnetic activity it is shown that the solar wind coupling parameter e and the magnetospheric output parameter UT are equivalent and that ranges of values of e can be set up in terms of values of the ring current-time constant τ. These conclusions were originally suggested by Akasofu (1981, Space Sci. Rev.28, 111).