Low Solar Wind Density Research Articles

Ground disturbances of the ring, magnetopause, and tail currents on the day the solar wind almost disappeared

We study ring current dynamics on a day of exceptionally low solar wind density (of ∼0.1 cm−3). Inner magnetospheric measurements of geomagnetic activity show geomagnetic quiet par excellence. On May 11, 1999, the values were |Dst| < 10 nT and Kp = 0+. The field at geostationary orbit was very close to dipolar. We simulate the global evolution of the ring current ion population during this period and compare the effect of the magnetopause, ring, magnetotail, and field‐aligned currents on the Dst index. Measured H+, He+, and O+ energy and pitch angle distributions by the Magnetospheric Ion Composition Spectrometer (MICS) and HYDRA instruments on Polar on May 9 are used as initial conditions for our kinetic model. Comparing model results with Polar data on May 11, we find remarkable agreement, proving the applicability of our model for these magnetospheric conditions. We compare modeled H+ and He+ ion distributions with quiet time ring current distributions inferred from statistical studies, and we find that as a general trend, the simulation results have lower values than the statistical patterns. The ground magnetic field disturbances due to the ring and the magnetopause currents decrease quasi‐monotonically on May 11, reaching limiting magnitudes of ∼5 and ∼3 nT, respectively. These values are substantially smaller than the ∼20 nT quiet time values obtained from statistical studies.

The location of low Mach number bow shocks at Earth

On April 26 and 27 and May 10–12, 1999, unusually low solar wind densities produced unusually low Alfven Mach numbers that moved the Earth's bow shock far out past its normal location. The shocks observed by the Wind spacecraft corresponded to shock subsolar distances of 45 and 42 RE, respectively, on these days which are the most distant locations at which the shock has ever been seen. Shock observations by three other spacecraft on these days along with 34 previously reported distant shocks are used to compare with the predictions of different models. A recent MHD bow shock model of Cairns and Lyon [1995] predicts the observed locations quite well as does a modified gasdynamic model of Farris and Russell [1994] if a new Mach‐number‐dependent shape parameter is used. A third model of Verigin et al. [1997] also predicts a shock shape and is also quite good. Bow shock predictions are limited by uncertainties in measurements of the very low densities and uncertainties in the position and shape of the magnetopause. Asymmetries in the shock shape caused by the interplanetary magnetic field direction and not accounted for by models are another likely source of uncertainty. These uncertainties make it impossible to clearly favor one theoretical model over another.

Statistical analysis of long-duration low-density solar wind events

Abstract. Low solar wind density with long duration was measured by in situ observation between 11 and 12 May 1999. As a result of this low-density solar wind condition, the magnetosphere of the Earth expanded considerably. We used a database of one-hour-averaged solar wind (1963–1999) near 1 AU to determine whether or not the observed low-density event was extremely abnormal. As a result it was found that this event has the longest duration in approximately 36 years of solar wind observations. There are three events with density 0.5 cm-3 or less and duration ten hours or longer. They were observed on 4 and 31 July 1979, and 11–12 May 1999. The 4 July 1979 event recurred on 31 July 1979. The events were characterized by low-beta, low Alfven Mach number (MA ), and low dynamic pressure. The occurrence rate of low-density solar wind with density 0.5 cm-3 or less shows several peaks near solar maxima. However, it is difficult to find a clear relationship between the sunspot number and the occurrence rate.Key words. Interplanetary physics (flare and stream dynamics; solar wind plasma; sources of the solar wind)

Polar cap X‐rays and electrons under low density solar wind conditions: Coordinated PIXIE and DMSP observations on 11 May 1999

X‐ray images from 11 May 1999 typically show emissions filling a region above about 75°–80° magnetic latitude with the emitting region centered a few degrees toward mid‐morning from the magnetic pole during a period when the solar wind reached unusually low values. Ionospheric particle measurements show the entire northern polar cap illuminated by precipitating electrons during much of this time, while the southern polar cap was mostly “dark.” The precipitating electrons had multicomponent spectra, one component with characteristic energy ∼200 eV throughout the observation period, and others with time‐varying characteristic energies ranging between ∼3.5 keV and 10 keV, all components with spatial characteristics common to polar rain. Measurements in the solar wind also show similar multicomponent spectra and are relatively well correlated with observations of solar radio bursts and the polar‐cap x‐ray flux. We conclude that the higher energy components were associated with electrons accelerated in solar flares or coronal flare‐like events.

Solar‐cycle variation of low density solar wind during more than three solar cycles

The May, 1999 low density (<1 cm−3) solar wind interval is one of a series of intervals of low density solar wind which have been detected since in‐situ, near‐Earth observations began. Examining the NSSDC OMNI database since 1965, covering solar Cycles 20–23, we show that such intervals, which are also periods of unusually low mass flux and low dynamic pressure, occur most frequently around sunspot maximum and are rarer at solar minimum. The occurrence rate of low‐density plasma may be higher in weaker sunspot cycles (Cycle 20 and the current Cycle 23). Around two‐thirds of periods with densities ≤1 cm−3 are associated with transient solar wind structures, in particular with ejecta and post‐shock flows. The majority of other events are associated with corotating streams. The May 1999 event is unusual because it is not associated with an ejecta or stream. A similar period was observed in July–August 1979.

Electrons in the low‐density solar wind

The recent occurrence of an interval (May 9–12, 1999) of abnormally low density solar wind has drawn attention to such events. The Solar Wind Experiment (SWE) instrument on the Wind spacecraft observed nine similar events between launch (November 1994) and August 1999: one in 1997, three in 1998, and five in January–August 1999. No such events were observed in 1996, the year of solar minimum. This already suggests a strong dependence upon solar activity. In this paper we discuss observations of the electron strahl, a strong anisotropy in the solar wind electrons above 60 eV directed along the magnetic field and observed continuously during the periods of low density in 1998 and 1999. When the solar wind density was less than 2 cm−3, the angular width of the strahl was below 3.5° and the temperature deduced from the slope of the electron strahl phase density (as a function of energy) in the energy range 200–800 eV was 100–150 eV, equivalent to a typical coronal electron temperature. Three examples of this phenomenon, observed on February 20–22, April 26–27, and May 9–12, 1999, are discussed to show their similarity to one another. These electron observations are interpreted to show that the strahl occurs as a result of the conservation of the first adiabatic invariant, combined with the lack of coulomb collisions as suggested by Fairfield and Scudder [1985].

GEOTAIL observations of anomalously low density plasma in the magnetosheath

During the anomalously low density solar wind interval of May 1999, GEOTAIL was in the magnetosheath for ∼37 hours after making an inbound crossing of the expanding bow shock at ∼8 RE upstream of its nominal position. Comparison among data sets obtained from GEOTAIL (magnetosheath), WIND (near upstream‐bow shock), and ACE (far upstream) reveals several unique features: Firstly, during the interval of 1430–1530 UT on 11 May, we observed both in the solar wind and magnetosheath double‐peaked protons with a peak separation of 250–300 km/s, which was close to the local Alfvén velocity during the event. Secondly, we observed extremely strong strahl electrons both in the solar wind and magnetosheath during the interval of 0600–2100 UT on 11 May 1999. We present an overview of the GEOTAIL observations, and discuss their physical significance.

Geomagnetic disturbances at high latitudes during very low solar wind density event

In this letter, we report geomagnetic observations over Greenland and Antarctica during May 11, 1999, when the solar wind almost disappeared. Greenland magnetometers show no magnetic activity in the nominal auroral zone but disturbances reached ∼200 nT at higher latitudes; however, conjugate observations in Antarctica show much weaker activity ∼40 nT. Analyzed data provide evidence that the cusp current system could be extended far to the dusk (dawn) in the northern (southern) polar region. This is consistent with the Ørsted satellite observations, where field‐aligned currents were detected near cusp, over the northern geomagnetic pole, and in the nightside polar cap; however, almost no were detected over the southern polar cap. We conclude that the low‐density solar wind quieted activity at auroral latitudes (”loading‐unloading” processes); but the “directly‐driven” convection observed over both polar caps was mainly unaffected by the significant inflation of the magnetosphere.

Magnetosheath electrons in anomalously low density solar wind observed by Geotail

We present an overview of the magnetosheath electron observations above 60 eV in the anomalously low density solar wind on 11 May 1999 observed by Geotail. We observed anisotropic ‘strahl’ component not only in the solar wind but also in the magnetosheath. Strahl is observed during the interval of 6–21h UT and most enhanced in the interval surrounding the solar wind density minimum in 18–19h UT. Its temperature seems positively correlated to its flux. We also evaluate the density and temperature of low energy isotropic component. The variation of density is associated with proton density. Its temperature is continuous, and suddenly decreased after the density minimum in ∼18–19h UT. We also compared our downstream data with upstream one observed by the ACE spacecraft. We conclude that the electron acceleration and heating at the bow shock is executed in the ordinal mechanisms even in the low density solar wind.

The Day the Solar Wind Almost Disappeared

On 11 May 1999, the flux of the solar wind, which carries charged particles away from the sun, decreased to very low values for an unusually long time. In this Perspective, [Lazarus][1] recounts the discussions of this event at a recent meeting. The reasons for low-density solar wind periods are not known, but detailed observations of its interactions with Earth's magnetic field were obtained. The event also provided an opportunity to obtain fundamental insights into the solar corona where the solar wind originates. [1]: http://www.sciencemag.org/cgi/content/full/287/5461/2172

Structure within Jovian hectometric radiation

Observations of Jovian hectometric radio emission (HOM) by the Voyager planetary radio astronomy (PRA) experiment at frequencies from 300 kHz to 1.3 MHz indicate persistent dynamic spectral features that had not been previously studied. The features of interest appear as “lanes” of decreased emission intensity within the otherwise persistent HOM. The lanes are apparent in intensity and occurrence probability spectrograms of frequency versus Jovian System III (1965) longitude. In the investigation of the morphology of these features, we use inbound and outbound Voyager 2 data at Jupiter to show that the lane occurrence and characteristics do not depend on local time over the range sampled. Occurrence probability spectrograms of frequency versus magnetic latitude are created from the portion of the data when the spacecraft was between 0° and +10° magnetic latitude. These spectrograms represent both the inbound and outbound passes and are quite similar despite the different longitude ranges. A simple extension of decametric (DAM) arc features into the HOM wavelength does not account for all the lane features, giving further evidence that HOM is an independent emission component. Polarization signatures for the data show that the polarization is predominantly right‐hand circular and that it does not reverse across the lanes, suggesting the emission is from the same hemisphere. In addition, we investigate possible effects due to solar wind variations and find that the occurrence of the lanes appears to be independent of times of low and high solar wind densities. The intensity of the HOM emission on either side of the lanes is comparable, implying that the lane is probably not a result of a gap between fundamental and second harmonic emission regions. We present these data and analyses as a morphological study to establish that the lane features are an important part of the HOM emission and should be considered in HOM emission models. At this time, no theory of the source of the lanes explains all the observed features.

Ulysses solar wind observations to 56� south

In the 25 months since Jupiter flyby, the Ulysses spacecraft has climbed southward to a heliolatitude of 56°. This transit has been marked by an evolution from slow, dense coronal streamer belt solar wind through two regions where the rotation of the Sun carried Ulysses back and forth between streamer belt and polar coronal hole flows, and finally into a region of essentially continuous fast, low density solar wind from the southern polar coronal hole. Throughout these large changes, the momentum flux normalized to 1 AU displays very little systematic variation. In addition, the bulk properties of the polar coronal hole solar wind are quite similar to those observed in high speed streams in the ecliptic plane at 1 AU. Coronal mass ejections and forward and reverse shocks associated with corotating interaction regions have also been observed at higher heliolatitudes, however they are seen less frequently with increasing southern heliolatitude. Ulysses has thus far collected data from 20° of nearly contiguous solar wind flows from the polar coronal hole. We examine these data for characteristic variations with heliolatitude and find that the bulk properties in general show very little systematic variation across the southern polar coronal hole so far.

Meteoric ions in the corona and solar wind

The total mass of refractory material of interplanetary origin penetrating and evaporated in the meltosphere surrounding the sun has been inferred from observations of meteoroids and fireballs falling in earth's atmosphere. The amount of iron atoms deposited this way in the solar corona is of the order of 3000 t/s or larger. The measured flux of outflowing solar wind iron ions is equal to 2200 t/s. The close agreement of both fluxes is evidence that a significant fraction of iron ions observed in the solar wind and in the corona must be of meteoric origin. A similar accord is also obtained for silicon ions. The mean velocity of meteoroid ions formed in the solar corona is equal to the free-fall velocity: i.e., independent of their atomic mass as the thermal speed of heavy ion measured in low-density solar wind streams at 1 AU. Furthermore, the heavy ions of meteoric origin escape out of the corona with a larger bulk velocity than the protons which are mainly of solar origin. These differences of heavy ion and proton bulk velocities are also observed in the solar wind. 52 refs.

Energetic particle events near the earth after the flare of 24 April 1985

Phenomena in the interplanetary space near the bow shock between 24 April – 2 May are discussed using data from Prognoz 10 and IMP 8. A complex structure in the solar wind and the interplanetary magnetic field passed the Earth after the solar flare of 24 April 1985. A high speed low density solar wind stream caused the long lasting expansion of the bow shock to distances more than 32 R E at the dusk flank. An enhancement of energetic electrons, protons and α-particles above the ambient solar particles was observed at the same time in a wide energy range. A similar expansion of the bow shock connected with a magnetic cloud was observed in the apogee of the next orbit (1–2 May) but no accelerated particles were registered in this event.