J. K. Arballo

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 relationship between interplanetary discontinuities and Alfven waves: Ulysses observations

The rate of occurrence of interplanetary discontinuities (ROID) is examined using Ulysses magnetic field and plasma data from 1 to 5 AU radial distance from the Sun and at high heliographic latitudes. We find two regions where the ROID is high: in stream-stream interaction regions and in Alfven wave trains. This latter feature is particularly obvious at high latitudes when Ulysses enters a high speed stream associated with a polar coronal hole. These streams are characterized by the presence of continuous, large-amplitude (ΔB→/|B|∼1−2)Alfven waves and an extraordinarily high ROID value (∼150 discontinuities/day). In a number of intervals examined, it is found that (rotational) discontinuities are an integral part of the Alfven waves. The nonlinear Alfven waves are spherically polarized, i.e., the tip of the perturbation vector resides on the surface of a sphere (a consequence of constant |B|). The slowly rotating part of the wave rotates ∼270° in phase. There is a slight arc in the B1-B2 hodogram, suggesting an almost linear polarization. The phase rotation associated with the discontinuity is ∼90°, lies in the same plane as the slowly rotating part of the Alfven wave, and therefore completes the 360° phase rotation. The best description of the overall Alfven wave plus discontinuity is a spherical, arc-polarized, phase-steepened wave.

Magnetic cloud field intensities and solar wind velocities

For the sets of magnetic clouds studied in this work we have shown the existence of a relationship between their peak magnetic field strength and peak velocity values, with a clear tendency that clouds which move at higher speeds also possess higher core magnetic field strengths. This result suggests a possible intrinsic property of magnetic clouds and also implies a geophysical consequence. The relatively low field strengths at low velocities is presumably the cause of the lack of intense storms during low speed ejecta. There is also an indication that this type of behavior is peculiar for magnetic clouds, whereas other types of non cloud-driver gas events do not seem to show a similar relationship, at least for the data studied in this paper. We suggest that a field/speed relationship for magnetic clouds, as that obtained in our present study, could he associated with the cloud release and acceleration mechanism at the sun. Since for magnetic clouds the total field tyically has a substantial southward component, B s , our results imply that the interplanetary dawn-dusk electric field, given by v X B s (where v is the clouds velocity), is enhanced by both factors. Therefore, the consequent magnetospheric energization (that is governed by this electric field) becomes more efficient for the occurrence, of magnetic storms.

Large Amplitude IMF Fluctuations in Corotating Interaction Regions: Ulysses at Midlatitudes

Corotating Interaction Regions (CIRs), formed by high-speed corotating streams interacting with slow speed streams, have been examined from −20° to −36° heliolatitudes. The high-speed streams emanate from a polar coronal hole that Ulysses eventually becomes fully embedded in as it travels towards the south pole. We find that the trailing portion of the CIR, from the interface surface (IF) to the reverse shock (RS), contains both large amplitude transverse fluctuations and magnitude fluctuations. Similar fluctuations have been previously noted to exist within CIRs detected in the ecliptic plane, but their existence has not been explained. The normalized magnetic field component variances within this portion of the CIR and in the trailing high-speed stream are approximately the same, indicating that the fluctuations in the CIR are compressed Alfven waves. Mirror mode structures with lower intensities are also observed in the trailing portion of the CIR, presumably generated from a local instability driven by free energy associated with compression of the high-speed solar wind plasma. The mixture of these two modes (compressed Alfven waves and mirror modes) plus other modes generated by three wave processes (wave-shock interactions) lead to a lower Alfvenicity within the trailing portion of the CIR than in the high-speed stream proper. The results presented in this paper suggest a mechanism for generation of large amplitude Bz fluctuations within CIRs. Such phenomena have been noted to be responsible for the generation of moderate geomagnetic storms during the declining phase of the solar cycle.

Auroral zone dayside precipitation during magnetic storm initial phases

Abstract Significant charged-particle precipitation occurs in the dayside auroral zone during and after interplanetary shock impingements on the Earths magnetosphere. The precipitation intensities and spatial and temporal evolution are discussed. Although the post-shock energy flux (10– 20 erg cm −2 s −1 ) is lower than that of substorms, the total energy deposition rate may be considerably greater (∼ an order of magnitude) than nightside energy rates due to the greater area of the dayside portion of the auroral oval (defined as extending from 03 MLT through noon to 21 MLT). This dayside precipitation represents direct solar wind energy input into the magnetosphere/ionosphere system. The exact mechanisms for particle energization and precipitation into the ionosphere are not known at this time. Different mechanisms are probably occurring during different portions of the storm initial phase. Immediately after shock compression of the magnetosphere, possible precipitation-related mechanisms are: (1) betatron compression of preexisting outer zone magnetospheric particles. The anisotropic plasma is unstable to loss-cone instabilities, leading to plasma wave growth, resonant particle pitch-angle scattering and electron and proton losses into the upper ionosphere. (2) The compression of the magnetosphere can also lead to enhanced field-aligned currents and the formation of dayside double-layers. Finally (3) in the latter stages of the storm initial phase, there is evidence for a long-lasting viscous-like interaction occurring on the flanks of the magnetopause. Ground-based observations identifying the types of dayside auroral forms would be extremely useful in identifying the specific solar wind energy transfer mechanisms.

Interplanetary Shocks, Magnetopause Boundary Layers and Dayside Auroras: The Importance of a Very Small Magnetospheric Region

Dayside near-polar auroral brightenings occur when interplanetary shocks impinge upon the Earths magnetosphere. The aurora first brightens near local noon and then propagates toward dawn and dusk along the auroral oval. The propagation speed of this wave of auroral light is ∼10 km s-1 in the ionosphere. This speed is comparable to the solar wind speed along the outer magnetosphere. The fundamental shock-magnetospheric interaction occurs at the magnetopause and its boundary layer. Several physical mechanisms transferring energy from the solar wind directly to the magnetosphere and from the magnetosphere to the ionosphere are reviewed. The same physical processes can occur at other solar system magnetospheres. We use the Haerendel (1994) formulation to estimate the acceleration of energetic electrons to 50 keV in the Jovian magnetosphere/ionosphere. Auroral brightenings by shocks could be used as technique to discover planets in other stellar systems.

Interplanetary discontinuities and Alfvén waves at high heliographic latitudes : Ulysses

This paper presents the results of the first statistical study of interplanetary directional discontinuities at both low and high heliographic latitudes measured by the Ulysses magnetometer. There is a gradual decrease in the rate of occurrence of interplanetary discontinuities (ROIDs) with increasing radial distance. From 1 to 5 AU, an e -(r-1)/5 dependence is derived. Much of this decrease is believed to be an artifact due to the discontinuity thickening with decreasing IBI, falling outside the detection criteria. As Ulysses goes from the ecliptic plane to high (-80°) heliographic latitudes, the ROID value increases dramatically. The increase is about a factor of 5 as Ulysses moves from Jupiter at 5 AU to 2.5 AU over the south pole. There is a one-to-one correspondence between high ROID values and high-speed streams. This is particularly dramatic just after the Jovian encounter when there are ∼25.4-day period corotating streams present. Thus the increase with latitude is primarily due to Ulysses spending an increasing percentage of time within a high-speed stream emanating from the solar coronal hole. High-speed streams are characterized by the presence of nonlinear Alfven waves with peak-to-peak transverse fluctuations as large as |ΔB|/| B| of 1 to 2. Over the south pole, the normalized transverse wave power can be characterized by P = 2.5 × 10 -4 f -1.6 Hz -1 and the compressional power 1.8 × 10 -4 f -1.2 Hz -1 for frequencies between 10 -5 and 10 -2 Hz. The normalized wave power spectra in different regions of the polar coronal hole streams, from midlatitudes to high heliographic latitudes, appear to be quite similar. The wave power in the ecliptic plane is somewhat lower, perhaps due to contamination from low-speed streams. The Alfven waves in the high-speed stream are found to be propagating outward from the Sum, even at these large heliocentric distances (2.5-5.0 AU). The waves typically have arclike polarizations and conserve field magnitude to first order. Directional (rotational) discontinuities often form the edges of the phase-steepened Alfven waves, thus offering a natural explanation for the high ROID rates within high-speed streams.

Interplanetary Causes of Great and Superintense Magnetic Storms

Abstract We examine possible interplanetary mechanisms for the creation of the largest magnetic storms at the Earth. We consider the effects of interplanetary shock events on magnetic cloud and sheath plasma. We also examine the effects of a combination of a long-duration southward sheath magnetic field, followed by a magnetic cloud BS event. Examination of profiles of the most intense storms from 1957 to the present indicate that the latter (double IMF BZ events) is an important cause of superintense DST events.

Broadband plasma waves observed in the polar cap boundary layer: Polar

Polar observations indicate the presence of intense broadband plasma waves nearly all of the time (96% occurrence frequency in this study) near the apogee of the Polar trajectory (∼6–8 RE). The region of wave activity bounds the dayside (0500 to 1800 LT) polar cap magnetic fields, and we thus call these waves polar cap boundary layer (PCBL) waves. The waves are spiky signals spanning a broad frequency range from ∼101 to 2 × 104 Hz. The waves have a rough power law spectral shape. The wave magnetic component has on average a ƒ−2.7 frequency dependence and appears to have an upper frequency cutoff of ∼(6–7) × 103 Hz, which is the electron cyclotron frequency. The electric component has on average a ƒ−2.2 frequency dependence and extends up to ∼2 × 104 Hz. The frequency dependences of the waves and the amplitude ratios of B′/E′ indicate a possible mixture of obliquely propagating electromagnetic whistler mode waves plus electrostatic waves. There are no clear intensity peaks in either the magnetic or electric spectra which can identify the plasma instability responsible for the generation of the PCBL waves. The wave character (spiky nature, frequency dependence and admixture of electromagnetic and electrostatic components) and intensity are quite similar to those of the low-latitude boundary layer (LLBL) waves detected at and inside the low-latitude dayside magnetopause. Because of the location of the PCBL waves just inside the polar cap magnetic field lines, it is natural to assume that these waves are occurring on the same magnetic field lines as the LLBL waves, but at lower altitudes. Because of the similar wave intensities at both locations and the occurrence at all local times, we rule out an ionospheric source. We also find a magnetosheath origin improbable. The most likely scenario is that the waves are locally generated by field-aligned currents or current gradients. We find a strong relationship between the presence of ionospheric and magnetosheath ions and the waves near the noon sector. These waves may thus be responsible for ion heating observed near the cusp region. Antisunward convection of these freshly accelerated oxygen ions over the polar cap during intense wave events (occurring during southward Bz events) might lead to enhanced plasma sheet O+ population. For magnetic storm intervals this mechanism would lead to a natural delay between the main phase onset and the appearance of oxygen ions in the ring-current.

Electromagnetic waves with frequencies near the local proton gyrofrequency: ISEE‐3 1 AU observations

Low Frequency (LF) electromagnetic waves with periods near the local proton gyrofrequency have been detected in interplanetary space by the magnetometer onboard ISEE-3. Transverse peak-to-peak amplitudes as large as ΔB→/||∼0.4 have been noted with compressional components (Δ |B|/ |B|) typically ≤ 0.1. Generally, the waves have even smaller amplitudes, or are not detectable within the solar wind turbulence. The waves are elliptically/linearly polarized and are often, but not always, found to propagate nearly along B→. Both right- and left-hand polarizations in the spacecraft-frame have been detected. The waves are observed during all orientations of the interplanetary magnetic field, with the Parker spiral orientation being the most common case. Because the waves are detected at and near the local proton cyclotron frequency, the generation mechanism must almost certainly be solar wind pickup of freshly created hydrogen ions. Possible sources for the hydrogen are the Earths atmosphere, coronal mass ejections from the Sun, comets and interstellar neutral atoms. At this time it is not obvious which potential source is the correct one. Statistical tests employing over one year of ISEE-3 data will be done in the near future to eliminate/confirm some of these possibilities.