3-5 Geomagnetic Storms NAGATSUMA Tsutomu Geomagnetic storms, in which the global geomagnetic field intensity decreases on the order of tens to hundreds nT, are phenomena that occur on the largest scale in the solar wind-magnetosphere-ionosphere coupled system. Geomagnetic storms develop when solar wind-magnetosphere couplings are intensified by solar wind disturbances (coronal holes and CME phenomena) that accompany southward IMF. Perturbations in the magnetic field are caused by geomagnetic storms and can be explained by the westward electric cur- rent along the geomagnetic equator (ring current). Such perturbations on the scale of 1015- 1016 J occur when the magnetosphere responds to the injections of energy during geomag- netic storms. Geomagnetic storms are generally believed to develop in association with an increase in magnetospheric convection. However, in contrast to magnetospheric convection develop- ment (which is saturated with strong solar wind electric fields), analysis of the correlation of solar wind parameters to magnetospheric convection and to geomagnetic storms has revealed that geomagnetic storm growth is not saturated with such electric fields. This indi- cates that geomagnetic field growth and magnetospheric convection growth may not corre- late perfectly. Keywords Geomagnetic storms, Solar wind-magnetosphere coupling, Magnetospheric convec- tion, Ring current, Substorm 1 Introduction currents in its fluid outer core. Couplings between this internal magnetic field and the The largest known disturbances in the solar wind form the magnetosphere, which has solar wind-magnetosphere-ionosphere coupled a comet-like shape, with a tail extending away system, geomagnetic storms are characterized from the Sun. Variations in solar wind-mag- by a prolonged period in which the horizontal netosphere couplings serve as the driving component of geomagnetic fields is depressed force behind various characteristic geomag- in the mid to low latitudes on a global scale, netic disturbances. Although components of with such periods lasting from one-half to sev- the geomagnetic field variation can be attrib- eral days. This paper examines the character- uted to the internal dynamo effect, their peri- istics of a geomagnetic storm and the cause of ods are much longer than the periods of varia- its development, as well as prospects for tions caused by solar wind-magnetosphere future research. couplings, and can be regarded as a separate phenomenon. 2 Features of Geomagnetic Vari- During quiet periods free of geomagnetic ations disturbances, the geomagnetic field displays a relatively regular pattern of variation. These The Earth has an internal magnetic field regular variations are caused by ionospheric generated by the dynamo effects of electric currents generated by the dynamo effect aris- NAGATSUMA Tsutomu 139 ing from the tidal motion of the thermosphere. solar wind, which is intensified by interplane- The tidal motion of the thermosphere is tary shock. driven by heat energy from the Sun. However, in certain cases, interplanetary Geomagnetic disturbances generally fall shock does not trigger geomagnetic storms. In into two broad categories. The first involves other words, geomagnetic perturbations during variations in the horizontal geomagnetic field the main and recovery phases are the essential component in the polar region in the range of features of a geomagnetic storm. These two several hundred to one thousand and several phases are believed to be essentially identical hundred nT, occurring on time scales of 30 in both SC and SG storm types. Substorms minutes to 2 hours. These variations corre- also frequently occur during geomagnetic spond to geomagnetic field perturbations storms, and the superposition of correspon- called "substorms." Optically, a substorm is ding short-period geomagnetic perturbations defined as a phenomenon that begins with an onto those of geomagnetic storms results in a explosive illumination of the aurora near the complex pattern of variations. midnight region on the nightside that gradual- The degree of depression of the horizontal ly expands both in the longitudinal and latitu- geomagnetic field component observed during dinal directions. The changes observed in the geomagnetic storms differs, depending on the geomagnetic field accompanying substorms magnetic local time. The maximum depres- differ significantly for different magnetic local sion of the geomagnetic field strength is seen times and magnetic latitudes. Furthermore, on the night to dusk side, while the minimum the effects of a substorm may appear as bay- depression is seen on the day to dawn side. like geomagnetic field variations in the low to This is due to the asymmetrical flow of the middle latitudes on the nightside. ring current, which will be explained in a later The second type of geomagnetic distur- section. bance involves a prolonged depression of the horizontal geomagnetic field component in the mid to low latitudes in the range of several tens to several hundred nT that lasts from one- half to several days. This type of disturbance is called a "geomagnetic storm." The period of progressive depression of field strength is called the "main phase." The period of restora- tion to original field strength is called the "recovery phase." A geomagnetic storm may accompany a "sudden commencement (SC)," characterized by a sudden increase in the mag- netic field intensity shortly before the main Fig.1 Examples of geomagnetic storms for phase. The period between the sudden com- sudden commencement (SC) type mencement and main phase is called the "ini- (top panel) and gradual (SG) type (bottom panel) tial phase." A geomagnetic storm not accom- panied by a SC is called a "gradual geomag- netic storm (SG)." Fig.1 shows an example of 3 Solar Wind Variations and Geo- SC- and SG-type geomagnetic storms magnetic Storms observed at the Kakioka Geomagnetic Obser- vatory. The SC is a geomagnetic perturbation Geomagnetic disturbances are driven by caused by an increase in magnetopause cur- solar wind-magnetosphere couplings. Solar rents due to the rapid compression of the mag- wind energy is injected into the magnetos- netosphere by the dynamic pressure of the phere through field line merging of the inter- 140 Journal of the Communications Research Laboratory Vol.49 No.3 2002 planetary magnetic field (IMF) and geomag- coronal mass ejection (CME) and coronal netic field. This energy injection is most effi- holes. CMEs are a phenomenon in which cient during southward IMF (Bs component). large amounts of solar coronal plasma is Prolonged periods of strong southward IMF released into interplanetary space (Fig.2). will trigger geomagnetic storms. Observa- CMEs appear simply as a region of high plas- tions have shown that a state of Bs >_ 10 nT ma density, but may contain a magnetic flux lasting for over 3 hours will always generate a rope structure[2]. geomagnetic storm[1]. Solar wind velocity The magnetic flux rope has a stable mag- (V) is another important factor. Geomagnetic netic field structure. When the structure con- storm development is known to demonstrate a tains a stable southward magnetic field com- strong positive correlation with the product of ponent, it is a significant driver of geomagnet- these two physical quantities, VBs. ic storms. Two types of solar surface phenomena are This structure also generates a interplane- believed to generate high VBs conditions: tary shock before the CME, helping to devel- Fig.2 CME observed by coronagraph (LASCO) aboard the SOHO satellite (ESA & NASA) NAGATSUMA Tsutomu 141 op geomagnetic storms when the magnetic storms may also be generated when CMEs field in the sheath region has a southward occur often in a specific active region on the component. Sun over several rotational periods. If a Coronal holes are areas in which high- strong southward magnetic field is produced speed solar winds stream out from the Sun. by interactions between a high-speed solar Coronal holes have weak solar magnetic wind and CME, even stronger geomagnetic fields compared to active regions and form in storms are triggered. regions having the same polarity over a wide area (unipolar magnetic fields). Such regions 4 Geomagnetic Storm Indices have lower coronal plasma temperatures than surrounding regions, and hence, appearing for Used as an indicator of the magnitude of a this reason to be relatively dark (Fig.3). The geomagnetic storm, the Dst index is based on high-speed solar winds ejected from coronal geomagnetic data at middle latitudes, with holes overtake and interact with low-speed ranges stretching in the meridional direction solar wind in its path, creating a co-rotating collected at four stations (Kakioka, Hermanus, interaction region (CIR). Within the CIR, San Juan, and Honolulu), and is expressed as plasma pressure is increased by compression. the hourly value indicating the degree of vari- Magnetic field strength also increases. ation in the symmetrical component[3]. The When the southward component is dominant locations of the geomagnetic observatories are in the strengthened magnetic field, its interac- shown in Fig.4. The Dst index is calculated tions with the geomagnetic field are strength- by assuming that the intensity of a geomagnet- ened, driving a geomagnetic storm. ic storm can be represented equivalently by Since CMEs are sporadic, geomagnetic the scale of the symmetric current flowing storms associated with them are sometimes westward at the equator, called the ring cur- called "sporadic geomagnetic