Substorm Energy Research Articles

Detection and Energy Dissipation of ULF Waves in the Polar Ionosphere: A Case Study Using the EISCAT Radar

AbstractUltra‐low frequency (ULF) waves transfer energy and momentum into the ionosphere‐thermosphere system. To quantify this energy, this paper first presents a new method to quantitatively detect ULF waves in Incoherent Scatter Radar (ISR) data based on 2D fast‐Fourier transforms and subsequent reconstruction of the wave. In parallel with other data sets, including optical, magnetometer, satellite, and models, we present the first full ionospheric energy dissipation rates for a ULF wave, split into electromagnetic (EM) and kinetic fluxes. The EM energy deposition is calculated from the use of the Poynting theorem, looking at Joule and frictional heating rates, where both rates show the same order of magnitude (1.24 × 1013 and 7.3 × 1012 J) respectively when integrated over the wave lifetime of 2 hr 15 min and an area of 4° magnetic latitude × 74° magnetic longitude. However, contrary to the common assumption that the EM flux is dominant, we determined the kinetic flux, to be almost equal in magnitude (8.7 × 1012 J). This indicates that previous papers might have underestimated the total energy dissipation by ULF waves. Compared to the substorm energy budget, we find that locally, the ULF wave event studied here makes up approximately 10% of a typical substorm cycle budget.

Auroral kilometric radiation—The electron cyclotron maser paradigm

Auroral kilometric radiation (AKR) is the paradigm of intense radio emission from planetary magnetospheres. Being close to the electron gyro frequency and/or its lower harmonics, its observation indicates the non-thermal state of the source plasma. Emission is produced when the plasma enters a state of energetic excitation which results in deformation of the electron distribution function. Under certain conditions this leads to “quasi-coherent” emission. It is believed that the weakly-relativistic electron-cyclotron-maser instability is responsible for this kind of radiation. Since energetically radio radiation normally is not of primary importance in the large-scale magnetospheric phenomena, AKR as such has, for the purposes of large-scale magnetospheric physics, become considered a marginal problem. Here this notion is questioned. AKR while applying to the auroral region mainly during magnetospherically disturbed times carries just a fraction of the total substorm energy. It is, however, of diagnostic power in the physics of the upper auroral ionosphere and Space Weather research. As a fundamental physical problem of generation of radiation in non-thermal plasmas it remains not resolved yet. Many questions have been left open even when dealing only with the electron-cyclotron-maser. These can advantageously be studied in the magnetosphere proper both by observation and theory, the only continuously accessible place in space. The most important are listet here with hint on how they should be attacked. Its value is to be sought in the role it should play in application to the other magnetized planets, extra-solar planets, and to strongly magnetized astronomical objects as an important tool to diagnose the matter state responsible for radiation in the radio frequency range beyond thermal, shock or synchrotron radiation.

Substorm Energy Transport From the Magnetotail to the Nightside Ionosphere

AbstractThe present study addresses the transport of energy from the nightside magnetosphere to the ionosphere during the substorm expansion phase. From the cross‐examination of the estimates of various ionospheric and magnetospheric processes, it is inferred that (1) Poynting flux from the tail lobes to the plasma sheet accounts for the ionospheric energy dissipation during the substorm expansion phase, suggesting that the lobe reconnection is the primary driver of the substorm energy transport to the ionosphere; (2) the area around the duskside poleward boundary of the auroral bulge, where auroral emission is intense throughout the expansion phase, accounts for >10%, likely a few tens of percent, of the ionospheric substorm energy dissipation, suggesting that the area is a crucial energy sink; and (3) kinetic energy carried by the bursty bulk flows is comparable to the energy deposited in association with the auroral streamers, and each bursty bulk flow accounts for ~1% of the total substorm energy deposition, which may sum up to 10% throughout the expansion phase. Points 2 and 3 imply that energy deposited to the ionosphere is transported mostly along, rather than across, the magnetic field lines in the plasma sheet. The responsible field‐aligned plasma flows may be generated as the perpendicular plasma motion creates field‐aligned pressure gradient in the presence of field line curvature. Energy flux associated with enhanced convection in the plasma sheet is statistically examined with the Geotail data set, which confirms that more energy is carried earthward by the field‐aligned flows than by the convection flows.

Electron Cooling and Isotropization during Magnetotail Current Sheet Thinning: Implications for Parallel Electric Fields

AbstractMagnetotail current sheet thinning is a distinctive feature of substorm growth phase, during which magnetic energy is stored in the magnetospheric lobes. Investigation of charged particle dynamics in such thinning current sheets is believed to be important for understanding the substorm energy storage and the current sheet destabilization responsible for substorm expansion phase onset. We use Time History of Events and Macroscale Interactions during Substorms (THEMIS) B and C observations in 2008 and 2009 at ~18 − 25 RE to show that during magnetotail current sheet thinning, the electron temperature decreases (cooling), and the parallel temperature decreases faster than the perpendicular temperature, leading to a decrease of the initially strong electron temperature anisotropy (isotropization). This isotropization cannot be explained by pure adiabatic cooling or by pitch angle scattering. We use test particle simulations to explore the mechanism responsible for the cooling and isotropization. We find that during the thinning, a fast decrease of a parallel electric field (directed toward the Earth) can speed up the electron parallel cooling, causing it to exceed the rate of perpendicular cooling, and thus lead to isotropization, consistent with observation. If the parallel electric field is too small or does not change fast enough, the electron parallel cooling is slower than the perpendicular cooling, so the parallel electron anisotropy grows, contrary to observation. The same isotropization can also be accomplished by an increasing parallel electric field directed toward the equatorial plane. Our study reveals the existence of a large‐scale parallel electric field, which plays an important role in magnetotail particle dynamics during the current sheet thinning process.

Kinetic analysis of the energy transport of bursty bulk flows in the plasma sheet

The energy transport of bursty bulk flows (BBFs) is very important to the understanding of substorm energy transport. Previous studies all use the MHD bulk parameters to calculate the energy flux density of BBFs. In this paper, we use the kinetic approach, i.e., ion velocity distribution function, to study the energy transport of an earthward bursty bulk flow observed by Cluster C1 on 30 July 2002. The earthward energy flux density calculated using kinetic approach QKx is obviously larger than that calculated using MHD bulk parameters QMHDx. The mean ratio QKx/QMHDx in the flow velocity range 200–800 km/s is 2.7, implying that the previous energy transport of BBF estimated using MHD approach is much underestimated. The underestimation results from the deviation of ion velocity distribution from ideal Maxwellian distribution. The energy transport of BBF is mainly provided by ions above 10 keV although their number density Nf is much smaller than the total ion number density N. The ratio QKx/QMHDx is basically proportional to the ratio N/Nf. The flow velocity v(E) increases with increasing energy. The ratio Nf/N is perfectly proportional to flow velocity Vx. A double ion component model is proposed to explain the above results. The increase of energy transport capability of BBF is important to understanding substorm energy transport. It is inferred that for a typical substorm, the ratio of the energy transport of BBF to the substorm energy consumption may increase from the previously estimated 5% to 34% or more.

Substorms during different storm phases

Abstract. After the deep solar minimum at the end of the solar cycle 23, a small magnetic storm occurred on 20–26 January 2010. The Dst (disturbance storm time) index reached the minimum of −38 nT on 20 January and the prolonged recovery that followed the main phase that lasted for about 6 days. In this study, we concentrate on three substorms that took place (1) just prior to the storm, (2) during the main phase of the storm, and (3) at the end of the recovery of the storm. We analyse the solar wind conditions from the solar wind monitoring spacecraft, the duration and intensity of the substorm events as well as the behaviour of the electrojet currents from the ground magnetometer measurements. We compare the precipitation characteristics of the three substorms. The results show that the F-region electron density enhancements and dominant green and red auroral emission of the substorm activity during the storm recovery resembles average isolated substorm precipitation. However, the energy dissipated, even at the very end of a prolonged storm recovery, is very large compared to the typical energy content of isolated substorms. In the case studied here, the dissipation of the excess energy is observed over a 3-h long period of several consecutive substorm intensifications. Our findings suggest that the substorm energy dissipation varies between the storm phases.

Can intense substorms occur under northward IMF conditions?

This paper reports on several substorms observed under northward interplanetary magnetic field (IMF) conditions, the intensity of which was at least as significant as that of typical substorms under moderately southward IMF conditions. Such northward IMF periods were identified during the recovery phase of three strong storms. In the case of each storm, two or more substorms occurred successively, being separated by ∼1.8–5 h, while the IMF condition continued persistently northward. The substorms are clearly evidenced by auroral and other complementary observations. For the most intense substorms, the auroral breakup occurred at the magnetic latitude of ∼58°, and for the others it was between 60° and 65°. The polar cap size prior to each onset was substantial despite the northward IMF conditions. The auroral expansion following each onset lasted from a few up to several magnetic local time hours and exhibited a clear poleward expansion feature. For most of the events studied, geosynchronous magnetic dipolarizations preceded by field stretching and/or energetic particle injections occurred. The occurrence of such (intense) substorms implies that a certain (large) amount of energy remains in the tail even under northward IMF conditions. The occurrence of two or more successive substorms further implies that even after the release of a certain amount of energy triggered by the substorm, the tail can still have a substantial amount of energy left, which can be released by a subsequent substorm(s). We conjecture that an intense substorm during a northward IMF period can be expected when such a period belongs to the recovery phase of an intense storm mainly because of large energy loading done by preceding southward IMF Bz during the storm's main (and some early recovery) phase. In addition we argue that substorm energy can also be supplied by other mechanisms of the solar wind–magnetosphere coupling under northward IMF conditions such as dayside reconnection in the presence of a substantial IMF By component.

Timing and location of substorm onsets from THEMIS satellite and ground based observations

Abstract. The unprecedented coverage of the THEMIS GBO station network coupled with high temporal and spatial resolution allowed us to determine the various stages of the global scale developments of the optical aurora at substorm onsets. We identified several steps of the substorm onset auroral phenomena and we suggest that the most rapid development is the starting of the Substorm Poleward Expansion (SPE) and it is most useful for accurate timing of the substorm onset. The physical significance of this step is the start of the large scale substorm energy dissipation in the atmosphere due to particle precipitation and auroral electrojet currents. We also recognized several pre-cursor features. We also measured the time of arrival of magnetic impulses associated with the same substorms at the THEMIS satellites. We used these times and a simple model with assumed iono-acoustic speeds in the range of 300–800 km/s to calculate the location and time of the origin of the magnetic impulses propagating from substorm onset. The assumption was made that the substorm occurred between two THEMIS satellites and the impulses propagated away from a singular starting point in and out along the magneto tail GSM-x axis. This technique is only useful in cases where the ground based signature of the substorm is very close in local time (or longitude) to the foot of the field lines of the THEMIS satellites. The x distance of the calculated origins were naturally highly dependent on the assumed propagation velocity model and the associated magneto-sonic speed. The resulting x distances of the starting point for the three events ranged between 11 and 17.6 RE. denoting a starting region that requires highly stretched field lines to map to the auroral onset latitude but which is generally considered to be too close for neutral line formation. The corresponding start times were in the range of 0 to 170 s prior SPE depending strongly on the assumed propagation speed.

Energy transfer in the solar wind–magnetosphere: Long-term fluctuations and intermittency

In this paper, we investigate the transfer of energy from the solar wind to the terrestrial magnetosphere. We make a selected brief review of the links between long-term fluctuations of the solar wind variables (velocity, magnetic field) and several multiscale magnetospheric processes. We emphasize those aspects that describe the non-linear magnetospheric response to solar wind changes. The variations of the flux of relativistic electrons with solar wind velocity over a solar cycle are discussed. Experimental evaluation of the substorm energy budget at solar maximum and minimum is also reviewed. The paper provides also a brief review on the intermittent fluctuations observed in the solar wind and the magnetosphere. The solar wind intermittency may be viewed as the hallmark of a turbulent process that sometimes departs from the traditional fluid or MHD turbulence. Recent analysis of geomagnetic indices and their correlation with the solar wind show that large amplitude, intermittent fluctuations have probabilities larger than for a Gaussian distribution, at smaller temporal scales. In the magnetosphere, particularly in the plasma sheet and the cusp the magnetic field fluctuations also show signs of intermittency that could be associated to a multifractal structure of the energy transfer. The dynamical aspects reviewed in this paper contribute to a picture of the magnetosphere as a non-linear system, driven out of equilibrium and responding to solar forcing on a broad range of spatio-temporal scales.

Среднеширотное проявление полярной суббури и реализация сейсмической энергии в зоне Вранча

Среднеширотное проявление полярной суббури и реализация сейсмической энергии в зоне Вранча

Study of the<tex>$rm Dst-AL$</tex>Correlation During Geospace Magnetic Storms

A dispute on the role of substorms in geospace magnetic storm development has emerged during the last decade; the scientific community has not yet reached a definite conclusion. This paper attempts to clarify, to the extent possible with statistical methods, if there is any causal relationship between the energy dissipation during storms and during substorms. To assess this aspect of the storm-substorm relationship, we performed an investigation using the geomagnetic indexes SYM-H and AL for a large number of magnetic storms. We selected a group of 53 magnetic storms with SYM-H peak values in the range between -50 nT and -350 nT. We then calculated the integrals of the SYM-H and AL squares, to represent magnetic energy dissipation for storms and substorms respectively. These integrals exhibit a strong linear correlation, with a correlation index of r=0.928. The result implies a strong coupling between storm and substorm energy dissipation.

Dependence of geosynchronous relativistic electron enhancements on geomagnetic parameters

Relativistic electron fluxes observed in geosynchronous orbit by GOES‐8 in 1997 to 2000 were considered as a complex function of geomagnetic indices PC, Kp, and Dst, as well as parameters of the magnetosphere size, subsolar Rs, and terminator Rf magnetopause distances. A geosynchronous relativistic electron enhancement (GREE) is determined as daily maximal electron flux exceeding the upper root mean square deviation (RMSD) threshold of about 1500 (cm2s sr)−1. Comparison analysis of the GREE dynamics and geomagnetic conditions on the rising phase of current solar cycle revealed suppression of the relativistic electron enhancements by substantially increased strong geomagnetic activity in the solar maximum. Statistical consideration of a relationship between the GREEs and the geomagnetic parameters showed that the most important parameters controlling the geosynchronous relativistic electron enhancements were 4‐day averaged Kp index, PC index, and magnetopause termination distance Rf, delayed on 3 and 14 hours, respectively. Relatively high averaging time for Kp was explained by the cumulative effect of substorm energy release in a gradual mechanism accelerating the relativistic electrons in the magnetosphere. Very short time delay for PC index was interpreted as intensification of a fast acceleration mechanism producing the GREEs during severe geomagnetic storms. Substantial increase of the PC index (PC > 5) was found to be a sufficient condition for GREE occurrence. The fast response of the geosynchronous relativistic electron fluxes on the magnetosphere compression was explained by drift losses of the energetic electrons at the magnetopause, which approaches the Earth during geomagnetic storms.

Magnetospheric energy budget and the epsilon parameter

Determination of the energy input for the magnetospheric energy budget is a nontrivial matter. As no direct means to measure the input are known, various solar wind‐derived proxies have been developed. In this article we discuss one of the most widely used energy input functions, the so‐called epsilon parameter of Akasofu. While practice has shown it to be a very useful parameter, there is no convincing evidence that it is superior to all other coupling parameters. Furthermore, its somewhat unclear definition and lack of physical foundation sometimes lead to confusing interpretation of the parameter in practical studies of magnetospheric energy cycle. For example, the parameter is sometimes understood to describe the transfer of solar wind Poynting flux into the magnetosphere, whereas the actual physical energy transfer involves conversion of solar wind kinetic energy to magnetic energy measured inside the magnetopause. Another questionable interpretation is to relate the size of the energy transfer region to the length of the reconnection line, as the scale factor in epsilon has the physical unit of area. These confusions may partly result from mixing the concepts of energy source and energy transfer. In spite of these problems the present empirical formulation of the epsilon parameter appears, from the global energy budget point of view, to give a remarkably good estimate for the total energy input into the inner magnetosphere in substorm and storm timescales. This is even more remarkable as after the parameter was first formulated we have learned that the ionosphere is a major sink of storm and substorm energy, exceeding the ring current in importance as an energy output channel. An additional issue is the energy carried away by the plasmoids and outflow of the postplasmoid plasma sheet. One can argue that the application of epsilon should be restricted to the energy consumption in the inner magnetosphere. However, as the intermittent plasmoid releases are essential parts of the same complex of processes as the ring current enhancement and ionospheric particle injections, we argue that they should be included in the energy budget, even if that might result in rejection of the epsilon as a useful input parameter. The recent analyses of energy output suggest that we can still use epsilon by scaling the parameter up by a factor of 1.5–2. It should be noted, however, that this energy budget does not account for all energy passing through the magnetosphere but only that part which is consumed in the storm and substorm processes.

Substorm energy budget during low and high solar activity: 1997 and 1999 compared

Substorms during the years 1997 and 1999 are analyzed to examine substorm energy budget just after solar minimum and prior to solar maximum. The energy input from the solar wind into the magnetosphere is estimated in terms of the time integral of Akasofu's epsilon parameter computed from Wind and ACE observations. The ionospheric Joule heating dissipation is estimated using the local electrojet index IL derived from the IMAGE magnetometer array of the MIRACLE ground‐based network in the Scandinavian sector. In total, 839 substorms from the midnight sector have been investigated to show that on average the Northern Hemisphere Joule heating accounts for ∼30% of solar wind energy input during 1997 and 1999 substorms. We found that during the active year 1999, there were 26% more substorm events, they were 15% more intense, and they were located at lower latitudes than during 1997. Isolated and stormtime substorms were also examined separately. Mean intensity of isolated substorms was about −350 nT, whereas it was about −670 nT for stormtime events. This study confirms our previous results (for 1997 only) that the amount of Joule dissipation depends on the energy input during the substorm expansion phase. Furthermore, the correlation is best for substorms recorded in the postmidnight sector, indicating that the energy budget and substorms size are largely controlled by processes driven directly by the solar wind.

Poynting vectors of Pc 5 pulsations observed by the GEOTAIL satellite in the dayside outer magnetosphere

Poynting vectors of Pc 5 pulsations observed in the dayside outer magnetosphere are examined with the magnetic and electric field data simultaneously observed by the GEOTAIL satellite. Poynting energy can be estimated as about 10−8–10−6 W/m2 in both directions across and along the magnetic field-line. However, the energy propagating across the magnetic field-line is larger than the energy propagating along the magnetic field-line. From this result we can estimate the Pc 5 wave energy flowing into the inner magnetosphere during an hour as 1010–1013 J, which is one or two orders less than the substorm energy (about 1014–1015 J). However, by taking into account of the continuous activation of Pc 5 pulsations in the outer magnetosphere, Pc 5 waves should play an important role for energetics in the magnetosphere.

The size of the polar cap as an indicator of substorm energy

The area of the polar cap, i.e. the region of open magnetic field lines or, more practically, the region inside the auroral-oval auroral distribution can be a good indicator of the amount of energy in the magnetotail available for magnetospheric substorms. It is shown that the polar cap is not a simple circle during the episodic development of substorms but expands and contracts non-uniformly, depending strongly upon magnetic local time. Variations of the polar cap area under such circumstances represent merely the average around the oval of different time changes in the polar cap boundary location at different local times, in response to solar wind conditions and the substorm phases. It is demonstrated that this non-uniform expansion/contraction of the polar cap can be accounted for in terms of the relative strength of the two constituents of the solar wind-magnetosphere coupling: the directly-driven and unloading processes.

A quantitative assessment of energy storage and release in the Earth's magnetotail

Magnetospheric substorms represent a global interaction between the solar wind, the magnetosphere, and the ionosphere. Energy extracted from the solar wind is episodically stored in the magnetosphere, with a large fraction of this energy being in the form of excess magnetic flux in the magnetotail lobes. The stored energy is periodically dissipated in an explosive instability that occurs in the near‐Earth plasma sheet at substorm expansive phase onset. Methods are discussed to estimate the amount of energy transferred from the solar wind to the magnetosphere during substorm growth phases. Observational and modeling constraints are then used to assess quantitatively the total amount of energy stored in the plasma sheet and magnetotail lobes. Possible energy sources to drive substorm evolution are evaluated in one particularly well‐observed case (May 3, 1986). By examining the major avenues of energy dissipation, the energy that is released in the form of plasma sheet heating, ionospheric Joule heating, etc. is assessed during the substorm. It is found that stored tail‐lobe energy is sufficient (by a factor of 3 or more) to drive observed substorm dissipation processes. On the other hand, energy in the closed field lines of the plasma sheet is insufficient (by a substantial margin) to supply the substorm energy.

Energy requirement of magnetic reconnection during magnetospheric substorms

Methods are discussed to estimate energy transfer from the solar wind to the magnetosphere during substorm growth phases. Observational and modeling constraints are then used to assess quantitatively the total amount of energy stored in the magnetotail. The major avenues of energy dissipation are examined and the energy that is released in the form of plasma sheet heating, ionospheric Joule heating, plasmoids, and energetic particle production during substorms is assessed. Energy sources are evaluated to drive substorm evolution in a particularly well-observed case: stored tail-lobe energy is sufficient to drive observed substorm dissipation processes (by large margins). On the other hand, energy in the closed field lines of the plasma sheet is insufficient to supply the substorm energy. Hence, magnetic reconnection is required on energy grounds during well-observed substorm cases.

Global variations of thermospheric winds and temperatures caused by substorm energy injection

Two numerical simulations of the thermospheric response to magnetospheric energy injection have been performed using a zonally averaged, time‐dependent model of neutral composition, dynamics, and energy budget. The simulations are distinguished by the duration of the source. The first simulation has an energy injection of 1 hour, representative of substorm type forcing, and the second one has a 12‐hour energy injection, representative of main storm type forcing. They were performed under the condition of equinox at solar minimum. In the first simulation, large‐scale atmospheric gravity waves (AGWs), generated by the substorm energy via Joule heating of ionospheric currents, are clearly identified in the wind‐field in a meridional plane as well as in the temporal and spacial variations of the total energy density of air above about 130 km height. These waves reach the equator after about 3 hours and propagate into the opposite hemisphere. The horizontal propagation speed is close to the speed of sound (for example, roughly 440 m/s at about 150 km altitude and 670 m/s at about 260 km altitude). Snapshots of the wind system affected by the substorm energy injection show a “four‐cell” pattern between the poles. Above 260 km, the cells have the opposite rotational direction to those below. These small‐scale features in the wind system are indicative of the internal atmospheric gravity waves with the vertical phase propagation. From a term analysis of the energy conservation equation, it is identified that the dominant energy process associated with the propagation of AGWs is adiabatic compressional heating and/or expansive cooling process. It can be concluded that the energy oscillations at middle and low latitude are mainly produced by AGWs propagating from high latitude during the substorm. The second simulation indicated that horizontal and vertical advections, vertical heat conduction, and infrared radiative cooling by nitric oxide are important in addition to adiabatic compressional heating and/or expansive cooling. It is suggested that short‐duration energy injection preferentially generates AGWs which dominate the energy oscillations at low latitudes through adiabatic heating and cooling. Long‐duration energy injection is more effective in generating a meridional circulation which transfers energy by both advective and adiabatic processes.

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 < −10 nT, typically the field directionality is highly variable, and large southward components have durations less than 3 hours. Thus the corotating stream/HCS plasma sheet interaction region can cause recurring moderate (−100 nT ≤ Dst ≤ −50 nT) to weak (−50 nT ≤ Dst ≤ −25 nT) storms, and sometimes no significant ring current activity at all (Dst > −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 Alfvén 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 Alfvén 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 (Alfvén 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.