I. G. Richardson

Solar and interplanetary sources of major geomagnetic storms (Dst ≤ −100 nT) during 1996–2005

[1] We present the results of an investigation of the sequence of events from the Sun to the Earth that ultimately led to the 88 major geomagnetic storms (defined by minimum Dst �� 100 nT) that occurred during 1996–2005. The results are achieved through cooperative efforts that originated at the Living with a Star (LWS) Coordinated DataAnalysis Workshop (CDAW) held at George Mason University in March 2005. On the basis of careful examination of the complete array of solar and in situ solar wind observations, we have identified and characterized, for each major geomagnetic storm, the overall solar-interplanetary (solar-IP) source type, the time, velocity, and angular width of the source coronal mass ejection (CME), the type and heliographic location of the solar source region, the structure of the transient solar wind flow with the storm-driving component specified, the arrival time of shock/disturbance, and the start and ending times of the corresponding IP CME (ICME). The storm-driving component, which possesses a prolonged and enhanced southward magnetic field (Bs), may be an ICME, the sheath of shocked plasma (SH) upstream of an ICME, a corotating interaction region (CIR), or a combination of these structures. We classify the Solar-IP sources into three broad types: (1) S-type, in which the storm is associated with a single ICME and a single CME at the Sun; (2) M-type, in which the storm is associated with a complex solar wind flow produced by multiple interacting ICMEs arising from multiple halo CMEs launched from the Sun in a short period; (3) C-type, in which the storm is associated with a CIR formed at the leading edge of a high-speed stream originating from a solar coronal hole (CH). For the 88 major storms, the S-type, M-type, and C-type events number 53 (60%), 24 (27%), and 11 (13%), respectively. For the 85 events for which the surface source regions could be investigated, 54 (63%) of the storms originated in solar active regions, 11 (13%) in quiet Sun regions associated with quiescent filaments or filament channels, and 11 (13%) were associated with coronal holes. Remarkably, nine (11%) CME-driven events showed no sign of eruptive features on the surface or in the low corona (e.g., no flare, no coronal dimming, and no loop arcade, etc.), even though all the available solar observations in a suitable time period were carefully examined. Thus while it is generally true that a major geomagnetic storm is more likely to be driven by a frontside fast halo CME associated with a major flare, our study indicates a broad distribution of source properties. The implications of the results for space weather forecasting are briefly discussed.

A study of an expanding interplanetary magnetic cloud and its interaction with the Earth's magnetosphere: The interplanetary aspect

In a series of three interlinked papers we present a study of an interplanetary magnetic cloud and its interaction with the Earths magnetosphere on January 14/15, 1988. This first paper is divided into three parts describing the principal results concerning the magnetic cloud. First, by applying the cylindrically symmetric, magnetic flux rope model to the high time resolution magnetic field and plasma data obtained by the IMP-8 spacecraft, we show that the axis of the magnetic cloud in question is approximately in the ecliptic and orthogonal to the Earth-Sun line. We note the presence of pulsations of ∼5-hour period in the bulk flow speed which are superimposed on an otherwise monotonically falling bulk speed profile. Second, we apply ideal MHD to model the self-similar, radial expansion of a magnetic cloud of cylindrical geometry. As initial condition for the magnetic field we choose a constant-α, force-free magnetic configuration. We demonstrate that the theoretical velocity profile for the free expansion of a magnetic cloud is consistent with observations made during the January 14/15, 1988, magnetic cloud encounter. Comparing model with data, we infer that prior to the start of observations at 1 AU the magnetic cloud had been expanding for 65.4 hours; the radius of the magnetic cloud at the time it arrived at Earth was 0.18 AU; and its expansion speed at 1 AU was ∼114 km/s. Third, we discuss energetic (∼1 MeV) ion data, also from instrumentation on IMP-8. We highlight the appearance of a sharp enhancement in the intensity of ∼0.5-MeV ions while IMP-8 was inside the cloud. These ions travel as a collimated, field-aligned beam from the west of the Sun. This is an “impulsive” solar event in which particles accelerated at a magnetically well-connected solar flare arrive promptly at the spacecraft. The observation of solar flare particles inside the cloud suggests that field lines within the magnetic cloud remained connected to the Sun. The observation is, however, inconsistent with the supposition that the cloud is formed of closed magnetic field loops disconnected from the Sun.

Sources of geomagnetic storms for solar minimum and maximum conditions during 1972-2000

We determine the solar wind structures (coronal mass ejection (CME)-related, corotating high-speed streams, and slow solar wind) driving geomagnetic storms of various strength over nearly three solar cycles (1972–2000). The most intense storms (defined by Kp) at both solar minimum and solar maximum are almost all (∼97%) generated by transient structures associated with CMEs. Weaker storms are preferentially associated with streams at solar minimum and with CMEs at solar maximum, reflecting the change in the structure of the solar wind between these phases of the solar cycle. Slow solar wind generates a small fraction of the weaker storms at solar minimum and maximum. We also determine the size distributions of Kp for each solar wind component.

Coronal mass ejections, interplanetary ejecta and geomagnetic storms

StudiesusingSOHOspacecraftdatahavedemon- strated that frontside halo coronal mass ejections (CMEs) detected by the LASCO coronagraphs generally precede ge- omagnetic storms. Nonetheless, about three quarters of such CMEs do not result in even moderate geomagnetic activity. We study the relationship of all the ejecta (in- terplanetary CMEs) which passed Earth during 1996-1999 to coronagraph CMEs and geomagnetic activity. We reach the following conclusions: (1) Only about half of frontside halo CMEs encounter the Earth; (2) The geoeectiveness of ejecta depends strongly on the southward magneticeld strengthand, for thesame southwardeld, is irrespective of whetherornottheejectahasamagneticcloudstructure;(3) Transit speeds of ejecta to Earth are only loosely correlated with CME speeds, one influence being the prevailing solar windconditions betweentheSunandEarth; (4)Ejecta may be detected at Earth even when there is no preceding halo CME observed by LASCO. Such ejecta are not particularly geoeective.

Sources of geomagnetic activity over the solar cycle: Relative importance of coronal mass ejections, high‐speed streams, and slow solar wind

We assess the contribution of various types of solar wind structures (coronal mass ejections (CMEs), high-speed streams, and slow solar wind) to averages of the aa geomagnetic activity index ( ) during the solar cycle. We used solar wind plasma, magnetic field, and energetic particle data to identify the flow types present in the near-Earth solar wind during 1972–1986 (encompassing the decline of solar cycle 20 and all of cycle 21). Corotating high-speed streams contribute ∼ 70% of outside of solar maximum and ∼ 30% at solar maximum (1978–1982). CME-related structures (shocks/postshock flows/ejecta) account for ∼ 50% of at solar maximum and <10% outside of maximum. Slow solar wind contributes ∼ 20% throughout the solar cycle. Our analysis provides insight into the cause of the so-called “Gnevyshev Gap” in geomagnetic activity, characterized by a decrease in near the peak of the sunspot cycle. An example of this phenomenon occurred in 1980 at the maximum of cycle 21 when registered a value lower than that observed at the preceding solar minimum. We attribute the 1980 depression in to a temporary reduction in average solar wind speed, evident in both CME and corotating stream related components, and a reduction in mean magnetic fields in all types of solar wind structure. This involvement of all solar wind structures is indicative of a global solar phenomenon, apparently related to an observed depression in the solar open magnetic flux at the time of solar magnetic field polarity reversal. Both CMEs and streams contribute to geomagnetic activity on either side of this minimum. Thus, at least for cycle 21, the Gnevyshev Gap does not reflect a transition between a period of enhanced geomagnetic activity levels due to CMEs just prior to solar maximum and a second enhancement, due to corotating streams, during the descending phase. Beyond the post Gnevyshev Gap peak, high-speed streams will eventually dominate geomagnetic activity on the decline of the cycle and may, on occasion (as in solar cycle 20), produce a late peak in average geomagnetic activity with relatively little contribution from CMEs.

Sources of geomagnetic activity during nearly three solar cycles (1972-2000)

mass ejections (CMEs), shocks, and postshock flows) to averages of the aa geomagnetic index and the interplanetary magnetic field (IMF) strength in 1972–2000 during nearly three solar cycles. A prime motivation is to understand the influence of solar cycle variations in solar wind structure on long-term (e.g., approximately annual) averages of these parameters. We show that high-speed streams account for approximately two-thirds of long-term aa averages at solar minimum, while at solar maximum, structures associated with transients make the largest contribution (50%), though contributions from streams and slow solar wind continue to be present. Similarly, high-speed streams are the principal contributor (55%) to solar minimum averages of the IMF, while transientrelated structures are the leading contributor (40%) at solar maximum. These differences between solar maximum and minimum reflect the changing structure of the near-ecliptic solar wind during the solar cycle. For minimum periods, the Earth is embedded in high-speed streams 55% of the time versus 35% for slow solar wind and 10% for CME-associated structures, while at solar maximum, typical percentages are as follows: high-speed streams 35%, slow solar wind 30%, and CME-associated 35%. These compositions show little cycle-to-cycle variation, at least for the interval considered in this paper. Despite the change in the occurrences of different types of solar wind over the solar cycle (and less significant changes from cycle to cycle), overall, variations in the averages of the aa index and IMF closely follow those in corotating streams. Considering solar cycle averages, we show that high-speed streams account for 44%, 48%, and 40% of the solar wind composition, aa, and the IMF strength,, respectively, with corresponding figures of 22%, 32%, and 25% for CME-related structures, and 33%, 19%, and 33% for slow solar wind. INDEXTERMS: 2134 Interplanetary Physics: Interplanetary magnetic fields; 2162 Interplanetary Physics: Solar cycle variations (7536); 2164 Interplanetary Physics: Solar wind plasma; 2111 Interplanetary Physics: Ejecta, driver gases, and magnetic clouds; 2788 Magnetospheric Physics: Storms and substorms; KEYWORDS: geomagnetic activity, solar cycle variation, solar wind, interplanetary magnetic field

Cosmic ray modulation and the solar magnetic field

We show that the variations of the interplanetary magnetic field strength (B) over a 22-year period are tracked by the inverted profile of the cosmic ray density measured by neutron monitors. We suggest that global changes in the Suns magnetic field are more important for long-term modulation than magnetic field enhancements resulting from the merging of high-speed flows and coronal mass ejections in the outer heliosphere. The unexpectedly close relationship that we find between the “tilt angle” of the heliospheric current sheet and the cosmic ray density away from solar minimum for both polarity states of the solar magnetic field may be accounted for by the anticorrelation between the cosmic ray density and field strength variations.

Iron charge distribution as an identifier of interplanetary coronal mass ejections

We present solar wind Fe charge state data measured on the Advanced Composition Explorer (ACE) from early 1998 to the middle of 2000. Average Fe charge states in the solar wind are typically around 9 to 11. However, deviations from these average charge states occur, including intervals with a large fraction of Fe 16 which are consistently associated with interplanetary coronal mass ejections (ICMEs). By studying the Fe charge state distribution we are able to extract coronal electron temperatures often exceeding 2 10 6 kelvins. We also discuss the temporal trends of these events, indicating the more frequent appearance of periods with high Fe charge states as solar activity increases.

Corotating MeV/amu ion enhancements at ≤1 AU from 1978 to 1986

MeV/amu ion enhancements associated with corotating high-speed solar wind streams in 1978–1986 during pre-solar maximum to near solar minimum conditions are studied using ISEE 3/ICE, IMP 8, and Helios 1 data. Around 50% of corotating streams contain energetic ion increases. These increases extend to ∼25 MeV/amu, where they merge into the galactic cosmic ray background, and are most evident approaching solar minimum. Sunward ion streaming in the solar wind frame (first-order anisotropy ∼20%) and positive radial intensity gradients (∼400%/AU) are consistent with acceleration in the outer heliosphere at corotating shocks followed by streaming into the inner heliosphere. The spectra and intensities show little solar cycle variation. The spectra of ions from protons to Fe at ∼2–20 MeV/amu are approximated equally well by exponentials in momentum dJ/dP ≈ exp (−P/P0), P0 = 11–16 MeV c−1 amu−1, or by distribution functions ƒ ≈ exp (−υ/υ0), υ0 = 0.18–0.25 (MeV/amu)1/2, with equivalent power law in energy slopes in the range ∼ −3 to −4. Ion abundances are correlated with the stream peak solar wind speed. In slower corotating streams (maximum solar wind speed <600 km/s), mean abundance ratios are protons/4He = 43 ± 18; 4He/O = 54 ± 23; C/O = 0.62 ± 0.06; Mg/O = 0.19 ± 0.03, and Fe/O = 0.14 ± 0.02. These show some similarity to the corresponding ratios for “solar energetic particles” (SEP) (protons/4He = 70 ± 10; 4He/O = 55 ± 3; C/O = 0.48 ± 0.02; Mg/O = 0.21 ± 0.01 and Fe/O = 0.16 ± 0.02) which are typically accelerated by shocks passing through slow solar wind. In corotating events in higher-speed streams, these ratios become protons/4He = 19 ± 5; 4He/O = 130 ± 35; C/O = 0.89 ± 0.05; Mg/O = 0.14 ± 0.01, and Fe/O = 0.10 ± 0.01 and more closely resemble the corotating event abundance ratios measured in high-speed streams during the mid-1970s solar minimum (protons/4He = 17 ± 7; 4He/O ∼ 160 ± 50; C/O = 0.89 ± 0.1; Mg/O = 0.13 ± 0.03, and Fe/O = 0.096 ± 0.05). Solar wind plasma may also show similar variations in composition with solar wind speed (based on the limited solar wind composition measurements available) so that the energetic ion compositions are consistent with the acceleration of corotating event ions and SEPs from the solar wind. The ordering of corotating event and solar wind abundances by first ionization potential and their variation with solar wind speed suggest that conditions in the ion-neutral fractionation region in the upper chromosphere determine the abundances and are associated in some way with regulation of the solar wind speed.

Cosmic ray decreases: 1964–1994

We have studied 30 years (1964–1994) of neutron monitor data in order to understand the principle mechanisms causing short-term (< 20-day duration) cosmic ray decreases seen at Earth. By examining the characteristics of associated low energy (<200 MeV) particle enhancements in combination with the neutron monitor data, we have determined the responsible solar wind disturbances for 153 of the 180 ≥ 4% decreases. The vast majority (86% of the 153 events) are caused by coronal mass ejections and the shocks that they generate. The ejecta is intercepted only when the solar event originates within 50° of the Suns central meridian. For more distant events, only the shock is intercepted at Earth. We present a list of all 180 events seen in the years 1964–1994 together with the associated solar event, when this can be determined, and some details about the solar wind structures based on in situ solar wind data, if available. This list represents a compendium of major solar wind disturbances affecting a large section of the inner heliosphere over this time period. We also discuss enhanced daily variations in some events which are related to radial gradients caused by strong disturbances inside the Earths orbit.