Frances Tang

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.

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.

Flux distribution of solar intranetwork magnetic fields

AbstractBig Bear deep magnetograms of June 4, 1992 provide unprecedented observations for direct measurements of solar intranetwork (IN) magnetic fields. More than 2500 individual IN elements and 500 network elements are identified and their magnetic flux measured in a quiet region of 300 × 235 arc sec. The analysis reveals the following results:(1)IN element flux ranges from 1016 Mx (detection limit) to 2 × 1018 Mx, with a peak flux distribution of 6 × 1016 Mx.(2)More than 20% of the total flux in this quiet region is in the form of IN elements at any given time.(3)Most IN elements appear as a cluster of mixed polarities from an emergence center (or centers) somewhere within the network interior.(4)The IN flux is smaller than the network flux by more than an order of magnitude. It has a uniform spatial distribution with equal amount of both polarities. It is speculated that IN fields are intrinsically different from network fields and may be generated from a different source as well.

Quiescent prominences - Where are they formed?

A survey of two years (1973 and 1979) of quiescent prominences reveals that substantially more (20 and 96% more, respectively, in the two years surveyed) quiescent prominences were formed on neutral lines between bipolar regions than on neutral lines inside bipolar regions.Present prominence models are based on the magnetic field configuration of the neutral lines of single bipolar regions, possibly because it is assumed that most prominences evolved from there. In view our new finding, a new model is needed in which the evolution begins at the boundary of two adjacent bipolar regions.

The velocities of intranetwork and network magnetic fields

We analyzed two sequences of quiet-Sun magnetograms obtained on June 4, 1992 and July 28, 1994. Both were observed during excellent seeing conditions such that the weak intranetwork (IN) fields are observed clearly during the entire periods. Using the local correlation tracking technique, we derived the horizontal velocity fields of IN and network magnetic fields. They consist of two components: (1) radial divergence flows which move IN fields from the network interior to the boundaries, and (2) lateral flows which move along the network boundaries and converge toward stronger magnetic elements. Furthermore, we constructed divergence maps based on horizonal velocities, which are a good representation of the vertical velocities of supergranules. For the June 4, 1992 data, the enhanced network area in the field of view has twice the flux density, 10% higher supergranular velocity and 20% larger cell sizes than the quiet, unenhanced network area. Based on the number densities and flow velocities of IN fields derived in this paper and a previous paper (Wang et al., 1995), we estimate that the lower limit of total energy released from the recycling of IN fields is 1.2 × 1028 erg s−1, which is comparable to the energy required for coronal heating.

A statistical study of active regions 1967–1981

We have studied 15 years of active region data based on the Mount Wilson daily magnetograms in the interval 1967–1981. The analysis revealed the following: (1) The integral number of regions decreases exponentially with increasing region sizes, or N(A) = 4788 exp(-A/175) for the 15 years of data, where A is the area in square degrees and N(A) is the number of active regions with area ≥A. (2) The average area of active regions varies with the phase of the solar cycle. There are more larger regions during maximum than during minimum. (3) Regions in the north are 10% larger on average than those in the south during this interval. This coincides with a similar asymmetry in the total magnetic flux between the hemispheres. (4) Regions of all sizes and magnetic complexities show the same characteristic latitude variation with phase in the solar cycle. The largest regions, however, show a narrower latitude range.

Motions, fields, and flares in the 1989 March active region

The results of observations of NOAA AR 5395 are presented. The region was observed every day from limb to limb for significant periods, and nine of the ten class-X flares were recorded. The region was found to be a great Delta group, dominated by spots of following (f) polarity, which moved rapidly westward, producing large changes in magnetic structure which increased the shear and led to great flares. Aside from its great size, the region was unusual in that normally p spots dominate and move westward. In this case there was a 4:1 flux imbalance; 80 percent of the flux measured was of following polarity. The major following spot in the region was found to move with a near-constant acceleration, eventually reaching 0.25 km/s. Rapid spot motion was discovered in all other superactive regions. Small p and f spots move out from either side of the large f spot, and curl around it in curved trajectories. The moving penumbral material coalesces into new umbrae.

The interplanetary and solar causes of geomagnetic activity

Abstract We present a review of recent work done on the topic of interplanetary and solar causes of geomagnetic activity. During solar maximum (1978–1979), 90% of the major magnetic storms ( D ST ⩽ − 100 nT) are caused by large southward B z events associated with interplanetary shocks. Of these, roughly half of the B z events are located in the sheath and half associated with the driver gas. These two sources of southward IMFs often give magnetic storms a two-step profile. The sheath field events are generated in the interplanetary medium between the outer corona and the Earth from the “shocking” of the slow solar wind upstream of the high speed stream. In contrast, the driver gas events are fields which come from the solar source region. A correlation between the field orientation at the solar source and that at 1 a.u. was sought, but none was found. Thus, quantitative predictions of storm intensities from solar observations appear to be very difficult. Prominence eruptions are shown to be an important cause of the high speed solar wind streams that lead to magnetic storms. The other 10% of the magnetic storms arc not related to interplanetary shocks or high speed streams, but to high density “non-compressional density enhancements”. Following magnetic storms arc “high-intensity long-duration AE activity events” (HILDCAAs) that are series of continuous auroral substorms that last from days to weeks during or after the storms recovery phase. HILDCAAs can also occur independently of magnetic storms. This continuous auroral activity is caused by the southward component of the magnetic field of interplanetary Alfven waves, presumably through the process of magnetic reconnection with the Earths field. These Alfven wave trains arc often observed in the trailing portions of high speed streams. From an analysis of a years data during solar maximum, it is found that the interplanetary medium is “Alfvenic” approx. 60% of the time. There appear to be no substantial differences in magnetusphcric response to Alfvenic or non-Alfvenic interplanetary intervals. The magnetopause boundary layer is shown to contain broad-band ELF/VLF plasma waves at least 85% of the time at all daysidc local times. These waves have sufficient amplitude to cause cross-field diffusion of magnetosheath plasma to form the low latitude boundary layer. Pitch angle scattering of the low latitude boundary layer particles is adequate to account for the dayside aurora. The only interplanetary /magnetosheath parameter that appears to affect the wave intensities is the IMF B z . Although the waves arc present at almost all times, they are intensified during southward IMF B z intervals.

Remote flare brightenings and type III reverse slope bursts

We present two large flares which were exceptional in that each produced an extensive chain of Hα emission patches in remote quiet regions more than 105 km away from the main flare site. They were also unusual in that a large group of the rare type III reverse slope bursts accompanied each flare.The observations suggest that this is no coincidence, but that the two phenomena are directly connected. The onset of about half of the remote Hα emission patches were found to be nearly simultaneous with RS bursts. One of the flares (August 26, 1979) was also observed in hard X-rays; the RS bursts occurred during hard X-ray spikes. For the other flare (June 16, 1973), soft X-ray filtergrams show coronal loops connecting from the main flare site to the remote Hα brightenings. There were no other flares in progress during either flare; this, along with the X-ray observations, indicates that the RS burst electrons were generated in these flares and not elsewhere on the Sun. The remote Hα brightenings were apparently not produced by a blast wave from the main flare; no Moreton waves were observed, and the spatially disordered development of the remote Hα chains is further evidence against a blast wave. From geometry, time and energy considerations we propose: (1) That the remote Hα brightenings were initiated by direct heating of the chromosphere by RS burst electrons traveling in closed magnetic loops connecting the flare site to the remote patches; and (2) that after onset, the brightenings were heated by thermal conduction by slower thermal electrons (kT∼1 keV) which immediately follow the RS burst electrons along the same loops.

Slow X-ray bursts and flares with filament disruption

The data from OGO-5 and OSO-7 X-ray experiments have been compared with optical data from six chromospheric flares with filament disruption associated with slow thermal X-ray bursts. Filament activation accompanied by a slight X-ray enhancement precedes the first evidence of Hα flare by a few minutes. Rapid increase of the soft X-ray flux accompanies the phase of fastest expansion of the filament. Plateau or slow decay phases in the X-ray flux are associated with slowing and termination of filament expansion. The soft X-ray flux increases as F∼(A + Bh) h, where h is the height of the disrupted prominence at any given time and A and B are constants. We suggest that the soft X-ray emission originates from a growing shell of roughly constant thickness of high-temperature plasma due to the compression of the coronal gas by the expanding prominence.