Range Of Heliolatitudes Research Articles

Magnetohydrodynamic Modeling of the Solar Wind Key Parameters and Current Sheets in the Heliosphere: Radial and Solar Cycle Evolution

Abstract We develop an axisymmetric numerical MHD model that allows us to investigate the spatial characteristics of the interplanetary magnetic field (IMF) and key solar wind plasma parameters from 20 to 400 solar radii over all heliolatitudes. The study is aimed at an analysis of the evolution of the spatial structure of the heliosphere through the solar cycle. We consider various combinations of the relative input of the quadrupole and dipole harmonics of the solar magnetic field to imitate the solar cycle. Self-consistent solutions for the IMF, electric current, solar wind speed, density, thermal pressure, and temperature in the solar wind are obtained. The spatial evolution of the IMF and properties of quasi-stationary current sheets (QCSs) are analyzed during different phases of the solar cycle. It is shown that a classic low-latitude heliospheric current sheet is formed in the solar wind as a part of the system of longitudinal and latitudinal electric currents symmetric in the northern and southern hemispheres only during solar minimum. While the quadrupole magnetic field increases, the second QCS appears. The model successfully describes a smooth transition from the state of the fast solar wind at high heliolatitudes and the slow solar wind at low heliolatitudes at solar minimum to the solar wind speed of the same values in a wide range of heliolatitudes at solar maximum. It reproduces the actively debated phenomenon of the south–north asymmetry of the IMF in the heliosphere and shows the distribution of thermal plasma parameters consistent with observations.

Studying the relationship of localization parameters of solar sources of magnetic clouds with their characteristics and substorm activity

We propose a method for determining location and orientation of extended solar sources of magnetic clouds, using coronagraph data and SOHO EIT/MDI images of the photosphere. To estimate the probability of formation of magnetic clouds, we use a simple cylindrical force-free model. We have established that more extended sources and those having a slight inclination to the solar equator and located on the solar limb as compared to those that are nonextended and strongly inclined can generate expanding clouds, which with high probability can reach the magnetosphere like clouds from a source near the zero meridian and low latitudes.
 We determine the relationship between extreme values of substorm activity and parameters of solar sources under study during the impact of magnetic clouds on Earth’s magnetosphere from the AL index. We note that there are no substorms associated with extended sources outside the heliolatitude range ~5–20°. The established relationship between solar source coordinates and geomagnetic activity of the magnetic cloud sheath and body are consistent with the most probable distribution of magnetoactive regions over the solar disk.

Исследование связи параметров локализации солнечных источников магнитных облаков с их характеристиками и суббуревой активностью

We propose a method for determining location and orientation of extended solar sources of magnetic clouds, using coronagraph data and SOHO EIT/MDI images of the photosphere. To estimate the probability of formation of magnetic clouds, we use a simple cylindrical force-free model. We have established that more extended sources and those having a slight inclination to the solar equator and located on the solar limb as compared to those that are nonextended and strongly inclined can generate expanding clouds, which with high probability can reach the magnetosphere like clouds from a source near the zero meridian and low latitudes.
 We determine the relationship between extreme values of substorm activity and parameters of solar sources under study during the impact of magnetic clouds on Earth’s magnetosphere from the AL index. We note that there are no substorms associated with extended sources outside the heliolatitude range ~5–20°. The established relationship between solar source coordinates and geomagnetic activity of the magnetic cloud sheath and body are consistent with the most probable distribution of magnetoactive regions over the solar disk.

Large-scale structure of solar wind and geomagnetic phenomena

In this work I propose a method for determining the solar wind parameters at distances from the Sun more than 1AU. The method is based on formulas for the scintillation indices and spectra for the interplanetary plasma with the large-scale flow structure. According to the results of observations of scintillations performed at the Ukrainian decameter radio telescope UTR-2, I analyzed the structure of the solar wind flows at different periods of the 23–24th solar cycles. The results of the work reveal that the speed of solar wind flows, which cause moderate geomagnetic storms with Ap≤66, was 590±40km/s. This is on average lower than that for the quasi-stationary fast solar wind, but higher than that for the slow solar wind. Solar wind disturbances which triggered geomagnetic storms showed a noticeable growth of solar wind velocity within a wider range of helio-latitudes comparing with disturbances which did not trigger geomagnetic storms. These results can be used to improve methods of forecasting magnetic storms on Earth.

Statistical study of low‐energy heliosphere particle fluxes from 1.4 to 5 AU over a solar cycle

Throughout the entire Ulysses mission, the Heliosphere Instrument for Spectra, Composition, and Anisotropy at Low Energies (HI‐SCALE) has collected measurements of low‐energy interplanetary ions and electrons. Time series of electron, proton, and ion fluxes have been obtained since 1990. We present statistical studies of high‐resolution ion and electron energy spectra (∼50 keV to ∼5 MeV) as measured by the HI‐SCALE instrument on the Ulysses spacecraft over a time interval longer than a solar cycle (1990 to 2004). Ulysses is the only spacecraft that continually measured the inner (∼1.4 to ∼5 AU) heliosphere particle population during these years. The data thus provide measures of the lower‐energy population of particles that a spacecraft traveling outward from Earth would have encountered and that also could have impacted the atmosphere and surface of Mars and of its satellites during this interval. Comparisons of Ulysses particle fluxes with those from the Electron, Proton, and Alpha Monitor (EPAM) instrument on the Advanced Composition Explorer (ACE) spacecraft (the HI‐SCALE backup instrument) have shown that azimuthal and heliolatitude dependencies of particle fluxes in the inner heliosphere following solar events are not as extreme as might be expected. Thus the Ulysses measurements, while taken over a range of heliolatitudes, can provide important statistical information that can be used to estimate the low‐energy radiation dosages and potential sputtering fluxes to planetary surfaces and to heliosphere spacecraft surfaces and solar arrays over a solar cycle.

Steady‐state event‐excluded proton spectra at solar minimum at all heliolatitudes

We present spectra of a steady foreground population of particles in the inner heliosphere observed by the Heliosphere Instrument for Spectra, Composition, and Anisotopy at Low Energies (HISCALE) on board the Ulysses spacecraft. The observed spectra come from the full range of heliolatitudes visited by Ulysses, −80 to +80 degrees, during Ulysses' first full orbit of the Sun during solar minimum. New corrections for background were applied resulting in a more sensitive and higher energy resolution data set that we used here to analyze quiet‐time (event‐excluded) spectra. We found a persistent foreground population of energetic protons that permeate the inner heliosphere. These protons have complex energy spectra suggesting multiple acceleration sources, and these particles move predominently in non‐radial directions. These findings support the idea that the inner heliosphere acts as a reservoir for energetic protons.

Study of solar features causing GMSs with 250γ<H<400γ

The effect of solar features on geospheric conditions leading to geomagnetic storms (GMSs) with planetary index,A P ≥ 20 and the range of horizontal component of the Earth’s magnetic fieldH such that 250γ <H < 400γ has been investigated using interplanetary magnetic field (IMF), solar wind plasma (SWP) and solar geophysical data (SGD) during the period 1978–99. Statistically, it is observed that maximum number of GMSs have occurred during the maximum solar activity years of 21st and 22nd solar cycles. A peculiar result has been observed during the years 1982, 1994 when sunspot numbers (SSNs) decrease very rapidly while numbers of GMSs increase. No distinct association between yearly occurrence of disturbed days and SSNs is observed. Maximum number of disturbed days have occurred during spring and rainy seasons showing a seasonal variation of disturbed days. No significant correlation between magnitude (intensity) of GMSs and importance ofH α , X-ray solar flares has been observed. Maximum number of GMSs is associated with solar flares of lower importance, i.e., SF during the period 1978-93.H α , X-ray solar flares occurred within lower helio-latitudes, i.e., (0–30)°N to (0–30)°S are associated with GMSs. NoH α , X-ray solar flares have occurred beyond 40°N or 40°S in association with GMSs. In helio-latitude range (10–40)°N to (10–40)°S, the 89.5% concentration of active prominences and disappearing filaments (APDFs) are associated with GMSs. Maximum number of GMSs are associated with solar flares. Coronal mass ejections (CMEs) are related with eruptive prominences, solar flares, type IV radio burst and they occur at low helio-latitude. It is observed that CMEs related GMS events are not always associated with high speed solar wind streams (HSSWSs). In many individual events, the travel time between the explosion on the Sun and maximum activity lies between 58 and 118 h causing GMSs at the Earth.

From the Sun’s south to the north pole – Ulysses COSPIN/LET composition measurements at solar maximum

Abstract. Based on elemental abundance ratios derived from the Ulysses COSPIN/LET measurements, we classified the energetic particle populations during and after the socalled Fast Latitude Scan – the time period during which the Ulysses spacecraft traveled from the highest heliolatitude south to maximum northern latitude, i.e. 27 November 2000 to 13 October 2001 – as being mixed between solar energetic particles (major component) and particles accelerated at stream interaction regions. During the fast latitude scan, the Ulysses spacecraft made the first transit in heliolatitude from pole to pole during solar activity maximum conditions, providing a unique opportunity to acquire energetic particle composition data over a maximum range of heliolatitudes in the inner heliosphere. At low latitudes, based on our elemental abundance analysis, we found that while solar energetic particles dominated, there were indications for particle acceleration at single compression regions in a few instances. In the high heliolatitude range the observed elemental particle compositions are mainly of the solar energetic particle type. Within the statistical errors, the observed abundance ratios were independent of latitude, and were characteristic of solar energetic particles. These observations raise an important question for the theories of particle propagation in the inner heliosphere. The daily elemental abundance ratios of S/O, Mg/O and Si/O shown here are the first measured ratios at high heliolatitudes in the energy range from 13.0 to 30.0 MeV/n.Key words. Interplanetary physics (energetic particles; interplanetary shocks) – Solar physics, astrophysics and astronomy (flares and mass ejections)

The global heliospheric magnetic field polarity distribution as seen at Ulysses

Abstract. The Ulysses spacecraft is in a near-polar solar orbit with a period of 6.2 years. The heliospheric magnetic field polarity detected by Ulysses from its 1992 Jupiter encounter to the current time is presented, following ballistic mapping of the polarity information to the solar wind source surface, at approximately 2.5 solar radii. The spacecraft’s first foray to polar latitudes and first rapid heliolatitude scan occurred in 1994–1995, near a minimum in solar activity. The heliospheric current sheet during this period was confined to low heliolatitudes. In 2000–2001, Ulysses returned in situ data from the same region of its orbit as in 1994–1995, but near to the maximum in solar activity. Unlike at solar minimum, heliospheric current sheet crossings were detected at the spacecraft over a wide heliolatitude range, which is consistent with the reversal of the solar magnetic dipole occurring during solar maximum. Despite complexity in the solar wind parameters during the latest fast latitude scan (McComas et al., 2002), the underlying magnetic field structure appears consistent with a simple dipole inclined at a large angle to the solar rotational axis. The most recent data show the heliospheric current sheet returning to lower heliolatitudes, indicating that the dipole and rotational axes are realigning, with the Sun’s magnetic polarity having reversed.Key words. Interplanetary physics (interplanetary magnetic fields; sources of the solar wind) – Solar physics, astrophysics and astronomy (magnetic fields)

A study of magnetic fluctuations and their anomalous scaling in the solar wind: the Ulysses fast-latitude scan

Abstract. The solar wind is a highly turbulent and intermittent medium at frequencies between 10-4 and 10-1 Hz. Power spectra are used to look at fluctuations in the components of the magnetic field at high frequencies over a wide range of latitudes. Results show steady turbulence in the polar regions of the Sun and a more varied environment in the equatorial region. The magnetic field fluctuations exhibit anomalous scaling at high frequencies. Various models have been proposed in an attempt to better understand the scaling nature of such fluctuations in neutral fluid turbulence. We have used the Ulysses fast latitude scan data to perform a wide ranging comparison of three such models on the solar wind magnetic field data: the well-known P model, in both its Kolmogorov and Kraichnan forms, the lognormal cascade model and a model adapted from atmospheric physics, the G infinity model. They were tested by using fits to graphs of the structure function exponents g(q), by making a comparison with a non-linear measure of the deviation of g(q) from the non-intermittent straight line, and by using extended self similarity technique, over a large range of helio-latitudes. Tests of all three models indicated a high level of intermittency in the fast solar wind, and showed a varied structure in the slow wind, with regions of apparently little intermittency next to regions of high intermittency, implying that the slow wind has no uniform origin. All but one of the models performed well, with the lognormal and Kolmogorov P model performing the best over all the tests, indicating that inhomogeneous energy transfer in the cascade is a good description. The Kraichnan model performed relatively poorly, and the overall results show that the Kraichnan model of turbulence is not well supported over the frequency and distance ranges of our data set. The G infinity model fitted the results surprisingly well and showed that there may very well be important universal geometrical aspects of intermittency over many physical systems.

Highlight Results from Ulysses

Launched in October 1990, the ESA-NASA Ulysses mission has conducted the very first survey of the heliosphere within 5 AU of the Sun over the full range of heliolatitudes. The first polar passes took place in 1994 and 1995, enabling Ulysses to characterise the global structure of the heliosphere at solar minimum, when the corona adopts its simplest configuration. The most important findings to date include a confirmation of the uniform nature of the high-speed (~ 750 km s−1) solar wind flow from the polar coronal holes, filling two-thirds of the volume of the inner heliosphere; the sharp boundary, existing from the chromosphere through the corona, between fast and slow solar wind streams; the latitude independence of the radial component of the heliospheric magnetic field; the lower-than-expected latitude gradient of galactic and anomalous cosmic rays; the continued existence of recurrent increases in the flux of low-energy ions and electrons up to the highest latitudes.

ORIGIN, INJECTION, AND ACCELERATION OF CIR PARTICLES: OBSERVATIONS Report of Working Group 6

This report emphasizes new observational aspects of CIR ions revealed by advanced instruments launched on the Ulysses, WIND, SOHO, and ACE spacecraft, and by the unique van- tage point of Ulysses which carried out the first survey of Corotating Interaction Region (CIR) properties over a very wide range of heliolatitudes. With this more complete observational picture established, this review is the basis to consider the status of theoretical models on origin, injection, and acceleration of CIR particles reported by Scholer, Mann et al. (1999) in this volume.

Coronal streamer belt asymmetries and seasonal solar wind variations deduced from Wind and Ulysses data

Solar wind measurements from Wind during March 1995 are combined with those from Ulysses' fast latitude scan to construct a map of the streamer belt. On the timescale of coronal change, the map is nearly a snapshot view of solar wind speed contours threaded by the trace of the heliospheric current sheet (HCS) in the ±30° heliolatitude range. The combined set of HCS crossings agrees remarkably well with the neutral line on the corresponding classical source surface map. The neutral line is displaced slightly southward, with latitudinal excursions ranging from −22° to +17°. In contrast, a line running through the locus of minimum speed, although following the general trend of the neutral line, is confined to lower latitudes and displaced slightly northward, ranging from −4° to +13°. The separation between the minimum speed locus and the neutral line marking the HCS was unexpected. Possible interplanetary and solar origins are discussed. The deduced asymmetries as well as coronal change between December and March were responsible for solar wind variations at Earth that mimicked the previously reported seasonal variation in Wind data and would have masked it had the observations been taken during September equinox.

Observations of evolving turbulence in the polar solar wind

The Ulysses magnetic field experiment has measured the heliospheric magnetic field in solar wind flows from the Sun's Southern polar coronal hole over a wide range of heliolatitudes and distances. We present an analysis of more than one year of data, covering a latitude range of 40° to 80°S and a distance range of 4.0 to 1.7 AU. We extend an earlier analysis of Ulysses magnetic field data and, by calculating structure functions, demonstrate that the spectrum of fluctuations in the magnetic field changed along the orbit of Ulysses within polar flows and that this change is a result of the variation in Ulysses' distance from the Sun, rather than its latitude. We interpret this change as being due to the development of turbulence in the polar solar wind.

Anomalous cosmic ray oxygen and neon (∼2.4 MeV/nucl) at high southern heliolatitudes

We report measurements of the anomalous oxygen and neon as measured with the HI‐SCALE instrument on Ulysses at high southern heliolatitudes. Using both energy spectral features and C/O ratios, 2.0–2.8 MeV/nucl anomalous O is not distinguishable below a latitude of ∼40°S. The heliolatitude gradient is found to be +0.06±0.50% / degree for the heliolatitude range 40°S to 80°S for an assumed radial gradient of 15% per AU. For the assumption of no radial gradient, the heliolatitude gradient is 0.39±0.42%/degree. The O flux at this energy is ∼2 × 10−8/(cm² sr s MeV/nucl); the Ne flux is ∼5 × 10−9/(cm² sr s MeV/nucl).

The ulysses mission — a voyage to the poles of the sun

The Ulysses space probe is the first heliospheric mission to carry out in situ observations over all solar latitudes. It is providing a unique set of fields and particles data which will allow a quantitative modelling of the threedimensional heliosphere. This paper reviews highlights of the observations made on board the Ulysses spacecraft over a range of heliolatitudes from near the ecliptic plane to the southern polar regions of the Sun.

Magnetic fields in the high latitude heliosphere

Observations in the out-of-ecliptic heliosphere had not been available before the Ulysses mission. Understanding the 3-dimensional structures and properties of the medium is important to model successfully those processes, such as the propagation and modulation of cosmic rays, which are expected to depend on heliolatitude. In this paper, magnetic field observations made onboard the Ulysses spacecraft over a range of heliolatitudes from near the ecliptic plane to the southern polar regions of the Sun are presented and discussed.

IPS measurements and azimuthal variation of solar-wind velocity on the heliospheric current sheet

The solar-wind velocity tends to be a minimum along the heliospheric current sheet and increases with heliomagnetic latitude. In addition, an azimuthal variation of solar-wind velocity is found to exist on the current sheet. In this study of the solar-wind velocity variations on the current sheet, we have made use of interplanetary scintillation (IPS) measurements. This method provides solar-wind data over a larger range of heliolatitudes: ± 80° or greater. We found that there exists a clear longitudinal variation of solar-wind velocity on the current sheet during 1973–1985. It is also noticeable that the pattern of variation changes with the phase of the solar cycle. On average, the solar-wind velocity varied from 300 to 550 km s −1 on the current sheet during the period of study. It is also found that the longitudinal variation of the solar-wind velocity on the current sheet obtained using near-Earth satellite data agrees well with that obtained using IPS data.

A test of magnetic field draping induced Bz perturbations ahead of fast coronal mass ejecta

Recent studies have suggested that the ambient interplanetary magnetic field (IMF) drapes about fast coronal mass ejections (CMEs) as they plow out through slower moving, quiescent solar wind. In this study we examine ISEE 3 plasma and magnetic field data in order to look for observational evidence of IMF draping ahead of fast CMEs. Since the upstream IMF is excluded from CMEs, the ambient field must drape about these ejecta and connect back to the Sun around their edges. For example, for a purely radial, inward directed IMF, this would mean that a CME which extends into the ecliptic plane while being directed northward of it should give rise to a southward Bz perturbation in the ecliptic plane ahead of the CME. While purely radial IMF configurations are rare, the radial component of the IMF should also give rise to such Bz perturbations. In reality, it is difficult to make an unambiguous identification of a CME's source region, and the draped field sampled along a spacecraft trajectory ahead of a CME can be very complicated, including contributions from a wide range of heliolatitudes as well as heliolongitudes. In spite of these complications we set out to test the usefulness of the draping scenario for predicting the Bz perturbations (and hence associated geomagnetic activity) ahead of fast CMEs which produce interplanetary shocks using ISEE 3 plasma and magnetic field data and Cane's (1985) list of shocks associated with interplanetary type II radio bursts. Of Cane's 45 events, there were 26 events which were suitable for testing the draping hypothesis. However, for only 17 of these 26 events was there a substantial change in the average Bz between the regions upstream from the shocks and between the shocks and CMEs. Of these 17 events the simpleminded radial component predictor developed in this study correctly predicts the direction of the Bz perturbations for 13 events (76%). While this result is certainly not conclusive, we consider it to be supportive of the draping scenario. In terms of using the draping model for real‐time geomagnetic activity predictions, however, the outlook is not nearly so good since nine of the 26 events had no substantial change in the average Bz and since there is no a priori way to tell how substantial draping effects will be for a given case.

Spatial variation and evolution of heliospheric sector structure

The magnetic sector polarity at the Sun and in interplanetary space has been surveyed during approximately three quarters of sunspot cycle 21 using photospheric magnetic field observations from the Wilcox Solar Observatory at Stanford University and spacecraft observations by Voyagers 1 and 2, Pioneer Venus Orbiter, and, to a lesser extent, Helios A, IMP 8, and ISEE 3. During the more than 8‐year period of the study, late 1977–1985, the Voyagers measured the interplanetary magnetic field (IMF) over a heliocentric distance range of 1–25 AU and a heliolatitude range of −5°–27°. The photospheric data were used together with a potential field (PF) model to calculate the location of the heliospheric current sheet near the Sun throughout the same period, which included sunspot maximum and much of the declining phase. The large‐scale magnetic polarity structure was found to be in almost continuous evolution throughout the period of study. This included an alternation between two‐sector and four‐sector patterns, with a mean duration in each case of approximately seven solar rotations. Although the azimuth of the IMF was found by the Voyager spacecraft to be oriented more than 60° from the theoretical Parker spiral angle 20% of the time, application of smoothing techniques produced IMF polarity patterns which could be compared with the pattern derived at the Sun. Such comparisons revealed very good agreement (82%) between solar and IMF sector patterns observed by Pioneer Venus in the inner heliosphere and significantly poorer agreement (61–64%) between solar and outer heliosphere patterns observed by Voyagers 1 and 2. This may be the result of dynamical processes in the solar wind which alter the position and shape of the current sheet beyond a few astronomical units. Sector pattern stability was found to decrease with increasing heliocentric distance, also. Evidence was found for the more frequent occurrence of four‐sector structure near the Sun than in the outer heliosphere. In 1985, when Voyager 1 reached a heliolatitude of 25°, it observed the disappearance of the sector structure, consistent with the tilt of the current sheet deduced from photospheric magnetic field measurements.