Bruce T. Tsurutani

What is a geomagnetic storm

After a brief review of magnetospheric and interplanetary phenomena for intervals with enhanced solar wind-magnetosphere interaction, an attempt is made to define a geomagnetic storm as an interval of time when a sufficiently intense and long-lasting interplanetary convection electric field leads, through a substantial energization in the magnetosphere-ionosphere system, to an intensified ring current sufficiently strong to exceed some key threshold of the quantifying storm time Dst index. The associated storm/substorm relationship problem is also reviewed. Although the physics of this relationship does not seem to be fully understood at this time, basic and fairly well established mechanisms of this relationship are presented and discussed. Finally, toward the advancement of geomagnetic storm research, some recommendations are given concerning future improvements in monitoring existing geomagnetic indices as well as the solar wind near Earth.

Dayside global ionospheric response to the major interplanetary events of October 29--30, 2003 ''Halloween Storms''

We demonstrate extreme ionospheric response to the large interplanetary electric fields during the Halloween storms that occurred on October 29 and 30, 2003. Within a few (2-5) hours of the time when the enhanced interplanetary electric field impinged on the magnetopause, dayside total electron content increases of ∼40% and ∼250% are observed for the October 29 and 30 events, respectively. During the Oct 30 event, ∼900% increases in electron content above the CHAMP satellite (∼400 km altitude) were observed at mid-latitudes (±30 degrees geomagnetic). The geomagnetic storm-time phenomenon of prompt penetration electric fields is a possible contributing cause of these electron content increases, producing dayside ionospheric uplift combined with equatorial plasma diffusion along magnetic field lines to higher latitudes, creating a daytime super-fountain effect.

Criteria of interplanetary parameters causing intense magnetic storms (Dst < −100 nT)

Abstract Ten intense magnetic storms (Dst 5 mV m−1, that last for intervals >3 h. Because we find a one-to-one relationship between these interplanetary events and intense storms, we suggest that these criteria can, in the future, be used as predictors of intense storms by an interplanetary monitor such as ISEE-3. These Bz events are found to occur in association with large amplitudes of the IMF (magnitude 13–30 nT) within 2 days after the onset of either high-speed solar wind streams or of solar wind density enhancement events, giving important clues to their interplanetary origin. Some obvious possibilities will be discussed. The close proximity of the Bz events and magnetic storms to the onset of high speed streams or density enhancement events is in sharp contrast to interplanetary Alfven waves and HILDCAA events previously reported by the authors (Tsurutani and Gonzalez, 1986, Planet. Space Sci.35, 405) and thus the two interplanetary features and corresponding geomagnetic responses can be thought of as being complementary in nature. An examination of opposite polarity (northward) Bz events with the same criteria (Bz > 10 nT, with dawn ward-electric fields >5 mV m−1, that last for intervals >3 h) shows that their occurrence is similar both in number as well as in their relationship to interplanetary disturbances, and that they lead to low levels of geomagnetic activity. Although 90% of the events were associated with high-speed streams and interplanetary shocks, the amplitude of the storms had little dependence on the strength of the shocks.

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.

The cause of high-intensity long-duration continuous AE activity (HILDCAAs): Interplanetary Alfvén wave trains

Abstract It is shown that high intensity (AE > 1,000 nT), long duration (T > 2d) continuous auroral activity (HILDCAA) events are caused by outward (from the sun) propagating interplanetary Alfven wave trains. The Alfven waves are often (but not always) detected several days after major interplanetary events, such as shocks and solar wind density enhancements. Presumably magnetic reconnection between the southward components of the Alfven wave magnetic fields and magnetospheric fields is the mechanism for transfer of solar wind energy to the magnetosphere. If the stringent requirements for HILDCAA events are relaxed, there are many more AE events of this type. A brief inspection indicates that these are also related to interplanetary Alfvenic fluctuations. We therefore suggest that most auroral activity may be caused by reconnection associated with Alfven waves in the interplanetary medium.

Great magnetic storms

The five largest magnetic storms that occurred between 1971 to 1986 are studied to determine their solar and interplanetary causes. All of the events are found to be associated with high speed solar wind streams led by collisionless shocks. The high speed streams are clearly related to identifiable solar flares. It is found that: 1) it is the extreme values of the southward interplanetary magnetic fields rather than solar wind speeds that are the primary causes of great magnetic storms, 2) shocked and draped sheath fields preceding the driver gas (magnetic cloud) are at least as effective in causing the onset of great magnetic storms (3 of 5 events) as the strong fields within the driver gas itself, and 3) precursor southward fields ahead of the high speed streams allow the shock compression mechanism (item 2) to be particularly geoeffective.

The Heliospheric Magnetic Field Over the South Polar Region of the Sun

Magnetic field measurements from the Ulysses space mission overthe south polar regions of the sun showed that the structure and properties of the three-dimensional heliosphere were determined by the fast solar wind flow and magnetic fields from the large coronal holes in the polar regions of the sun. This conclusion applies at the current, minimum phase of the 11-year solar activity cycle. Unexpectedly, the radial component of the magnetic field was independent of latitude. The high-latitude magnetic field deviated significantly from the expected Parker geometry, probably because of large amplitude transverse fluctuations. Low-frequency fluctuations had a high level of variance. The rate of occurrence of discontinuities also increased significantly at high latitudes.

Two‐step development of geomagnetic storms

Using the Dst index, more than 1200 geomagnetic storms, from weak to intense, spanning over three solar cycles have been examined statistically. Interplanetary magnetic field (IMF) and solar wind data have also been used in the study. It is found that for more than 50% of intense magnetic storms, the main phase undergoes a two-step growth in the ring current. That is, before the ring current has decayed significantly to the prestorm level, anew major particle injection occurs, leading to a further development of the ring current, and making Dst decrease a second time. Thus intense magnetic storms may often be the result of two closely spaced moderate storms. The corresponding signature in the interplanetary medium is the arrival of double-structured southward IMF at the magnetosphere.

Rapid Intensification and Propagation of the Dayside Aurora: Large Scale Interplanetary Pressure Pulses (fast shocks)

We present two cases of abrupt dayside auroral brightenings and very fast auroral propagation using the POLAR UV imaging data. The brightenings occur first at noon and then propagate along the auroral oval towards dawn and dusk. Ionospheric speeds of 6 to 11 km/s are determined. The auroral brightenings and motion are associated with the arrival and propagation of interplanetary shocks/pressure waves. The brightening at noon occurs within minutes of the shock compression of the noon-time magnetopause. The speed of the auroral propagation in the ionosphere towards dawn and dusk corresponds extremely well to the solar wind downstream flow. Our model assumes that shocks/pressure waves compress the outer dayside magnetosphere, and plasma contained therein. This plasma compression leads to the loss cone instability, wave-particle interactions, and concomitant particle loss into the ionosphere.

International Cometary Explorer encounter with Giacobini-Zinner - Magnetic field observations

The vector helium magnetometer on the International Cometary Explorer observed the magnetic fields induced by the interaction of comet Giacobini-Zinner with the solar wind. A magnetic tail was penetrated ∼7800 kilometers downstream from the comet and was found to be 104 kilometers wide. It consisted of two lobes, containing oppositely directed fields with strengths up to 60 nanoteslas, separated by a plasma sheet ∼103kilometers thick containing a thin current sheet. The magnetotail was enclosed in an extended ionosheath characterized by intense hydromagnetic turbulene and interplanetary fields draped around the comet. A distant bow wave, which may or may not have been a bow shock, was observed at both edges of the ionosheath. Weak turbulence was observed well upstream of the bow wave.