Interplanetary Disturbances Research Articles

Clementine particle measurements in lunar orbit

The Charge Particle Telescope (CPT) onboard the Clementine spacecraft measured the fluxes of energetic electrons and protons in near-Earth space during solar energetic particle events and magnetospheric disturbances. Protons in the energy range from 10 MeV to 80 MeV were of greatest interest for radiation effects such as total dose and single event upsets. CPT data were particularly useful in multi-spacecraft studies of in-terplanetary disturbances and their effects on the magnetosphere. The proton channels on the CPT provided measurements of solar energetic protons associated with the passage of an interplanetary shock at 0902 UT on 21 February 1994. The data provide good information concerning the shock passage timing and the energetic proton energy spectrum. Results are compared with those from GOES-7, SAMPEX, and GEOTAIL. The lower energy CPT electron channels (25–500 keV) were also of interest for magnetospheric studies. Although there was a significant solar interference problem, the CPT provided opportunities to observe energetic electron bursts during both magnetic storms and magnetospheric substorms.

Three‐dimensional MHD simulation of the April 14, 1994, interplanetary coronal mass ejection and its propagation to Earth and Ulysses

A three‐dimensional (3‐D), time‐dependent, magnetohydrodynamic (MHD) model is used to simulate the interplanetary propagation of a disturbance that started in the low corona via the destabilization of a southern hemisphere helmet‐streamer on April 14, 1994. A severe geomagnetic storm occurred at Earth, and a forward‐reverse shock structure was detected at Ulysses (3.2 AU) at E30°S60°. The model is initiated at 18Rs (where Rs is the solar radius, 6.95 × 105 km) within the supersonic and super‐Alfvenic region of the solar wind; hence no consideration is given to the disturbance's evolution from ∼ 1Rs to 18Rs. We refer to the interplanetary disturbance as an interplanetary/coronal mass ejection (ICME) to indicate that it may be relevant to what has been referred to in the literature as a CME but is not related to the flux rope or magnetic cloud that has also received much attention.We find that the 3‐D MHD model, with a simple pressure pulse (suggested by Yohkoh soft Xray observations), provides a satisfactory comparison with the SSC timing at Earth and the Ulysses observations.

Statistical nature of geomagnetic storms

On the basis of geomagnetic activity indices and solar wind parameters, a superposed epoch analysis has been conducted for more than 300 geomagnetic storms. The intensity of magnetic storms is found to depend on the duration of the main phase; larger storms have longer timescales. For intense storms, however, not only the duration of energy injection into the ring current but also the strength of injection is important in determining their size. It is confirmed that the southward component of the interplanetary magnetic field plays a crucial role both in triggering the storm main phase and in determining the magnitude of magnetic storms. It is also found that the time profile of the energy injection rate during the main phase tends to have two peaks. This is particularly the case for intense magnetic storms, where the second peak is more intense than the first. Implications of our findings are discussed in terms of the existing concept of geomagnetic storms and recent observations of ring current particles and of interplanetary disturbances.

The role of coronal mass ejections and interplanetary shocks in interplanetary magnetic field statistics and solar magnetic flux ejection

We examine the role of coronal mass ejections (CMEs) and interplanetary shocks in modifying the large‐scale winding of the interplanetary magnetic field (IMF) by extracting CME and shock observations from the ISEE 3 data set and analyzing periods of the disturbed and undisturbed solar wind separately. We use the full ISEE 3 data set representing the entire L1 mission (1978 – 1982). We conclude that CMEs, the shocks upstream of CMEs, and other interplanetary shocks are responsible for the apparent overwinding of the IMF spiral relative to the Parker prediction. The IMF winding angle asymmetry is preserved following the removal of the interplanetary disturbances. We also examine the IMF components, the IMF magnitude and the solar wind speed, and the dependence of those averages and asymmetries on CMEs and shock disturbances. An estimate is obtained for the anomalous azimuthal field contained within CMEs which apparently results from the closed‐field topology. We provide new evidence for a nonzero field component crossing the heliospheric current sheet. Last, we examine the role of CMEs and shocks in the measurement of solar magnetic flux ejection. We provide estimates for the average amount of flux transported by CMEs and the error in flux transport analyses that include shock data, and we examine the possible north‐south asymmetry of the flux.

A two‐dimensional simulation of the radial and latitudinal evolution of a solar wind disturbance driven by a fast, high‐pressure coronal mass ejection

Using a hydrodynamic simulation, we have studied the two‐dimensional (symmetry in the azimuthal direction) evolution of a fast, high‐pressure coronal mass ejection (CME) ejected into a solar wind with latitudinal variations similar to those observed by Ulysses. Specifically, the latitudinal structure of the ambient solar wind in the meridional plane is approximated by two zones: At low latitudes (< 20°) the solar wind is slow and dense, while at higher latitudes the solar wind is fast and tenuous. The CME is introduced into this ambient wind as a bell‐shaped pressure pulse in time, spanning from the equator to 45° with a speed and temperature equal to that of the high‐latitude solar wind. We find that such an ejection profile produces radically different disturbance profiles at low and high latitudes. In particular, the low‐latitude portion of the ejecta material drives a highly asymmetric disturbance because of the relative difference in speed between the fast CME and slower ambient solar wind ahead. In contrast, the high‐latitude portion of the same ejecta material drives a much more radially symmetric disturbance because the relative difference in pressure between the CME and ambient background plasma dominates the dynamics. The simulations reveal a number of other interesting features. First, there is significant distortion of the CME in the interplanetary medium. By ∼ 1 AU the CME has effectively separated (in radius as well as latitude) into two pieces. The radial separation is due to the strong velocity shear between the slow and fast ambient solar wind. The latitudinal separation arises from pressure gradients associated with rarefaction regions that develop as the CME propagates outward. Second, there is significant poleward motion of the highest‐latitude portion of the CME and its associated disturbance. The main body of the CME expands poleward by ∼ 18°, while the forward and reverse waves (produced by the overexpanding portion of the CME) propagate all the way to the pole. Third, the simulations show that the high‐pressure region, which develops at low latitudes as the fast CME ploughs through the slow ambient solar wind, penetrates significantly (∼ 10°) into the high‐latitude fast solar wind. We compare the simulation results with a CME‐driven interplanetary disturbance observed at both low and high latitudes and find that the simulation reproduces many of the essential features of the observations.

Travelling interplanetary disturbances detected using interplanetary scintillation at 327 MHz

Based on the advance predictions of two flare-generated shock fronts, obtained from the Space Environment Centre (SEC, NOAA, Boulder), observations of interplanetary scintillation (IPS) were carried out with the Ooty Radio Telescope (ORT) on a grid of appropriately located sources during the period 31 October to 5 November, 1992. Solar wind velocities were derived by fitting model spectra to the observed spectra and two travelling interplanetary disturbances were detected. Both disturbances were traced back to an active region on the Sun which was located close to a large coronal hole. The roles of flares and coronal holes in producing such disturbances are examined and it is shown that in the present case both the coronal hole and the active region probably played key roles in generating the two IPS disturbances.

Coronal magnetic storms: A new perspective on flares and the ?solar flare myth? debate

Recently Gosling (1993) examined the interplanetary consequences of solar activity, and suggested that the coronal mass ejection (CME) was the prime driver of most disturbances (i.e., interplanetary shocks, high-energy particles, geomagnetic storms, etc.) and that the solar flare was relatively unimportant in this context. He coined the phrase ‘Solar Flare Myth’. Since that paper there has been much debate on the origin of interplanetary disturbances - most people sitting squarely in the flare or CME camp. Svestka (1995) has attacked Gosling's conclusions on the grounds that it is misleading to ignore the flare, and that past flare classifications were perfectly adequate for explaining the observations described by Gosling. This paper is a comment on Svestka's report and an attempt to put the Solar Flare Myth into perspective - indeed it is an attempt to view the solar flare/CME phenomena in a more constructive light.

A large polar crown coronal mass ejection and a “problem” goemagnetic storm: April 14–23, 1994

In the 1960s and 1970s, systematic programs to associate geomagnetic storms with solar flares gave rise to a category of “problem storms” for which there were no clear associations or for which the associations were with solar activity that seemed too insignificant to properly account for the magnitude of the geomagnetic events. In the last 2 decades the role of filament eruptions and coronal mass ejections (CMEs) in driving geomagnetic activity has received increasing attention, and an answer to the problem storm question has been pieced together. Until recently the inability to view CMEs on the solar disk has impeded a precise demonstration of the inferred relationships. The data from the Yohkoh Soft X ray Telescope (SXT), however, allow the association of soft X ray coronal structures with the aftermath of CMEs. Since CMEs are now believed to be the main cause of aperiodic severe disturbances of Earth's magnetosphere, the ability to distinguish the aftermath of a CME on the disk improves our ability to predict geomagnetic storms. On April 14, 1994, a large scale coronal arcade formation was observed by SXT after the eruption of a long section of the southern polar crown neutral line. Data from the Mauna Loa coronagraph are consistent with this event being the result of a CME that formed in the helmet streamer underlying the heliospheric current sheet. The CME was followed by a severe geomagnetic storm, starting 68 hr later, and was also observed at 3.2 AU and high heliographic latitude by the Ulysses satellite. Since there was no significant flare associated with the eruption, and no significant filament disappearance, the subsequent geomagnetic events are an extreme example of a problem storm and of the great utility of soft X ray imagers for the prediction of space weather. In this paper we have assembled a wide range of data relating to the solar event, the evolution of the region that generated it, and the various observed consequences at Earth and in the interplanetary medium. The global magnetic field changes leading to the CME are outlined, a brief description of the coronal arcade formation is given, and the arcade's association with subsequent events at Earth and the interplanetary disturbance observed at Ulysses is described. We present this case as an example of the advantages to be derived from a comprehensive multidisciplinary approach to studying these potentially disruptive phenomena.

The distance to the solar wind termination shock in 1993 and 1994 from observations of anomalous cosmic rays

Using data from the Voyager and Pioneer spacecraft, we examine the energy spectra of anomalous cosmic rays (ACRs) during 1992–1994, a period of rapidly increasing intensities of these particles in the outer heliosphere. The 1992 period appears to be in nonequilibrium, marked by a rapid decrease in the tilt of the current sheet from ∼63° to ∼35° and a complex, evolving magnetic topology that affects interplanetary drifts and drift acceleration along the solar wind termination shock. In addition, large propagating interplanetary disturbances in 1991 may have weakened the shock temporarily, reducing the intensity of accelerated ions. The 1993–1994 period appears to be in quasi‐equilibrium, marked by a stable tilt of the current sheet and a steady, large‐scale magnetic topology that provides stable conditions for propagation and acceleration. The ACR intensities of higher‐rigidity particles rise slowly during this period, suggesting that the shock intensity has stabilized. At lower rigidities the ACR intensities are increasing rapidly, which we attribute to decreasing modulation. During 1994 days 157–209, we estimate that the shock was at 85 ± 5 AU. The shock strength s is estimated to be 2.63 ± 0.14, significantly weaker than a strong shock (s = 4) and consistent with a termination shock modified by the pressure of galactic cosmic rays. By comparing these 1994 observations with those made in 1987, we estimate that the latitudinal gradient of ACR He with energies 31–57 MeV nucleon−1 is ∼1.9%/deg along the shock from the equator to the pole. We estimate that the ratio of the efficiency of injection of He+ to H+ pickup ions into the acceleration process is 7.0 ± 1.6.

The use of interplanetary scintillation maps in the prediction of geomagnetic activity

Abstract. Interplanetary scintillation (IPS), the twinkling of small angular diameter radio sources, arises from the interaction of the signal with small-scale plasma irregularities in the solar wind. The technique may be used to sense remotely the near-Earth heliosphere and has potential for tracking large-scale interplanetary disturbances from close to the Sun to the Earth. Such observations might be useful within routine geomagnetic forecasts, and we use data from the Mullard Radio Astronomy Observatory to test this suggestion. A forecast was based on the visual evaluation of each daily map. If an IPS event was observed then we proposed that any associated geomagnetic activity would occur either on that day, or during the following two days. We consider the success of these forecasts in predicting days when either an SSC/SI or an Ap value exceeding 30 were recorded. The identification of IPS events is necessarily subjective and so two observers compiled independent events lists, and the results were compared. Approximately half of the IPS events in each list were followed by a geomagnetic signature but comparison of the two lists showed that different days were being chosen. We also found that the forecasts had very high false alarm rates. Since IPS is sensitive to a volume we did not expect all events to be associated with a geomagnetic signature. However, the technique failed to forecast a large proportion of geomagnetic events and the association between IPS events and geomagnetic activity is not much better than would be expected by chance. Comparing the IPS forecasts with forecasts of Ap released by the Space Environment Services Center (SESC) we found that SESC correctly predicted a similar proportion of days when Ap\\geq30, but that the performance was significantly better than would be expected by chance, and had a much lower false alarm rate. We conclude that these IPS data cannot be used alone to produce reliable geomagnetic activity forecasts.

Solar answers may be in the WIND

A monumental storm appeared before NASA's electronic eyes and then buffeted the Earth on October 18, 1995, but it wasn't a hurricane. Winds of a different sort—solar winds—delivered a monstrous magnetic cloud to Earth that day around 4 p.m. EST. The interplanetary disturbance smacked the “bow shock” at the sunward edge of Earth's magnetosphere and made it oscillate slightly, catalyzing a 2‐day display of magnetic storms and aurorae visible as far south as Denver. And NASA's WIND spacecraft was there to see it coming.

Study of Interplanetary Disturbances Using IPS

AbstractThe usefulness of Interplanetary Scintillation (IPS) data, which provide solar wind velocities (V) and relative scintillation indices (g), for predicting interplanetary disturbances is examined. Analysis of two years of g-map data from Cambridge shows that atleast two IPS stations are required for more definitive identification of events. Campaigns were made in April-May 1992 to map predicted solar wind disturbances simultaneously from two widely separated telescopes at Cambridge and Ooty. These show that apriori knowledge of strong flare activity helps in detecting scintillation enhancement. On the other hand, other events have been observed at Ooty, which show that both flares and coronal holes may be responsible for producing interplanetary disturbances, and hence it is premature to identify any one type of solar event as the sole cause of the disturbances.

Study of interplanetary disturbances using IPS

The usefulness of Interplanetary Scintillation (IPS) data, which provide solar wind velocities (V) and relative scintillation indices (g), for predicting interplanetary disturbances is examined. Analysis of two years of g-map data from Cambridge shows that atleast two IPS stations are required for more definitive identification of events. Campaigns were made in April-May 1992 to map predicted solar wind disturbances simultaneously from two widely separated telescopes at Cambridge and Ooty. These show that apriori knowledge of strong flare activity helps in detecting scintillation enhancement. On the other hand, other events have been observed at Ooty, which show that both flares and coronal holes may be responsible for producing, interplanetary disturbances, and hence it is premature to identify any one type of solar event as the sole cause of the disturbances.

Solar activity dependence of interplanetary disturbances

Interplanetary scintillation measurements obtained inside 200 R⊙ using the Ooty Radio Telescope during August 1986–April 1991 have been analysed to study the interplanetary disturbances (or events) and their occurrence rates at various phases of the solar cycle. The disturbances are identified by the increase in the level of scintillation compared with the expected value. In total, 735 events have been identified. The results show a rate of 0.49 events per day near solar maximum and a low rate of 0.16 events per day during minimum of activity. The results are compared with coronal mass ejection (CME) rates and transients rates obtained from the Doppler scintillation measurements.

Coronal and heliospheric observations of solar particle events

Observations of accelerated particle beams are used to probe the coronal and interplanetary magnetic field structures over large distances from the Sun on the order of a few AU and for various heliolatitudes. It is shown that the propagation of low energy particles is very much controled by discrete interplanetary magnetic field structures. These discrete magnetic structures are sometimes embedded within interplanetary solar wind plasma disturbances, commonly called CMEs. The connection between the corona and the interplanetary medium is discussed. These observations lead to new insights on the origin of accelerated particles detected in association with CMEs.

Solar Activity Dependence of Interplanetary Disturbances

AbstractInterplanetary scintillation measurements obtained inside 200 R⊙ using the Ooty Radio Telescope during August 1986 - April 1991 have been analysed to study the interplanetary disturbances (or events) and their occurrence rates at various phases of the solar cycle. The disturbances are identified by the increase in the level of scintillation compared with the expected value. In total, 735 events have been identified. The results show a rate of 0.49 events per day near solar maximum and a low rate of 0.16 events per day during minimum of activity. The results are compared with coronal mass ejection (CME) rates and transients rates obtained from the Doppler scintillation measurements.

IPS Observations of Short-Time Scale Interplanetary Activity

AbstractWe have carried out a program of continuous Interplanetary Scintillation (IPS) monitoring of the interplanetary activity using Ooty Radio Telescope (ORT). From May 1990 to March 1991, during the 22nd solar maximum, a few radio sources were monitored to provide long stretches of IPS data with a high-time resolution of few minutes. These observations covered 0.3 to 0.8 AU region (12° to 70° elongations) around the sun at several heliographic latitudes. During the observation, we detected 33 short-time scale IPS events which had significant variation in the scintillation index and solar wind velocity. These were considered to be due to travelling interplanetary disturbances.A multi-component model of plasma density enhancement was developed to estimate the geometry and physical properties of these IPS events. Detailed analysis of 20 of these events suggests, 1. fast IPS events were interplanetary signatures of Coronal Mass Ejections (CMEs), 2. the average mass and energy of these events was ~ 1016 gm and 1033 erg respectively, 3. 80% of IPS events were associated with X-ray flares on the sun and 50% were associated with geomagnetic activity at earth. Detailed study of the multi-component model suggests IPS observations at smaller elongations (hence at higher radio frequencies) are more suited to detect fast-moving interplanetary disturbances such as produced by CMEs.

IPS observations of short-time scale interplanetary activity

We have carried out a program of continuous Interplanetary Scintillation (IPS) monitoring of the interplanetary activity using Ooty Radio Telescope (ORT). From May 1990 to March 1991, during the 22nd, solar maximum, a few radio sources were monitored to provide long stretches of IPS data with a high-time resolution of few minutes. These observations covered 0.3 to 0.8 AU region (12° to 70° elongations) around the sun at several heliographic latitudes. During the observation, we detected 33 short-time scale IPS events which had significant variation in the scintillation index and solar wind velocity. These were considered to be due to travelling interplanetary disturbances.

Charged-particle telescope experiment on Clementine

The charged-particle telescope (CPT) onboard the Clementine spacecraft measured the fluxes of energetic protons emitted in solar energetic particle events. Protons in the energy range from 10 to 80 MeV were of greatest interest for radiation effects such as total dose and single event upsets. Energetic electrons were also of interest for spacecraft charging and their contribution to total dose. The lower-energy CPT electron channels (25-500 keV) were mainly of geophysical interest. While orbiting the moon, the CPT observed the wake created by the moon when it blocked the flow of energetic particles in the magnetotail region. The CPT provided opportunities to observe energetic electron bursts during magnetic storms and magnetospheric substorms. CPT data are particularly useful in multispacecraft studies of interplanetary disturbances and their interaction with the magnetosphere. The proton channels on the CPT provided data on solar energetic protons and storm-time protons associated with the passage of an interplanetary shock at 0903 UT on Feb. 21, 1994. Results are compared with those from GOES-7, SAMPEX, and GEOTAIL.

Density fluctuations in different types of solar wind flow at 1 AU and comparison with results from Doppler scintillation measurements near the Sun

Density fluctuations with periods in the range 10 min to 1 hour are investigated using ISEE 3 plasma measurements of solar wind flows at 1 AU. Coronal hole, interstream, plasma sheet, coronal mass ejection, and interaction region flow types are considered. The ISEE 3 results are consistent with the interpretation of large‐scale variations in density fluctuations observed by Doppler scintillation measurements inside 0.2 AU. The highest absolute and fractional density fluctuations are found ahead of and within the plasma from coronal mass ejections, with the maximum values occurring between the associated interplanetary shocks and the driver gas. For the quasi‐stationary solar wind, density and fractional density fluctuations are highest and most variable in the plasma sheet in which the heliospheric current sheet is embedded, and lowest and least variable in the high‐speed coronal hole flow. A superposed epoch analysis of heliospheric current sheet crossings shows that the regions of enhanced density fluctuations previously identified in Doppler scintillation measurements near the Sun persist to 1 AU and are broader there. The results of this study confirm the advantages of using density fluctuations rather than density as a tracer of solar wind flows of differing origins at the Sun and as a detector of propagating interplanetary disturbances.