L. Zhao

Global distribution of the solar wind during solar cycle 23: ACE observations

[1] The composition of the solar wind can be used to determine its origin at the Sun; e.g., solar wind from coronal holes has demonstrably lower charge states than solar wind of other origins. The 0 7+ /0 6+ ratio as measured by Advanced Composition Explorer (ACE) during 1998-2008 is used to divide the solar wind into three categories: non-transient solar wind from coronal holes (hereafter referred to as CHW); non-transient solar wind that originates from outside of coronal holes (hereafter referred to as NCHW), and solar wind associated with transient interplanetary coronal mass ejections (ICMEs). The global distribution of the solar wind relative to the Heliospheric Current Sheet (HCS), as specified by a Potential-Field-Source-Surface model, is then determined. The solar wind from outside of coronal holes is found to originate from a band of about 40° in width about the HCS during solar maximum conditions, and a much smaller band of < 17° during solar minimum. These results are consistent with models for the global transport of the solar magnetic field during the solar cycle, and they are consistent with earlier global flow structure determinations based upon velocity alone.

Tracking Solar Active Region Outflow Plasma from Its Source to the Near-Earth Environment

Seeking to establish whether active-region upflow material contributes to the slow solar wind, we examine in detail the plasma upflows from Active Region (AR) 10978, which crossed the Sun’s disc in the interval 8 to 16 December 2007 during Carrington rotation (CR) 2064. In previous work, using data from the Hinode/EUV Imaging Spectrometer, upflow velocity evolution was extensively studied as the region crossed the disc, while a linear force-free-field magnetic extrapolation was used to confirm aspects of the velocity evolution and to establish the presence of quasi-separatrix layers at the upflow source areas. The plasma properties, temperature, density, and first ionisation potential bias [FIP-bias] were measured with the spectrometer during the disc passage of the active region. Global potential-field source-surface (PFSS) models showed that AR 10978 was completely covered by the closed field of a helmet streamer that is part of the streamer belt. Therefore it is not clear how any of the upflowing AR-associated plasma could reach the source surface at 2.5 R⊙ and contribute to the slow solar wind. However, a detailed examination of solar-wind in-situ data obtained by the Advanced Composition Explorer (ACE) spacecraft at the L1 point shows that increases in O7+/O6+, C6+/C5+, and Fe/O – a FIP-bias proxy – are present before the heliospheric current-sheet crossing. These increases, along with an accompanying reduction in proton velocity and an increase in density are characteristic of both AR and slow-solar-wind plasma. Finally, we describe a two-step reconnection process by which some of the upflowing plasma from the AR might reach the heliosphere.

The heliospheric magnetic field and the solar wind during the solar cycle

The heliospheric magnetic field and the solar wind are behaving differently in the current solar minimum, compared to the previous minimum. The radial component of the heliospheric magnetic field, and thus the average value of the component of the solar magnetic field that opens into the heliosphere, the so-called open magnetic flux of the Sun, is lower than it was in the previous solar minimum; in fact, lower than in any previous solar minimum for which there are good spacecraft observations. The mass flux, the ram pressure, and the coronal electron temperature as measured by solar wind charge states are also lower in the current minimum compared to the previous one. This situation provides an opportunity to test some of the concepts for the behavior of the heliospheric magnetic field and the solar wind that have been developed; to improve these theories, and to construct a theory for the solar wind that accounts for the observed behavior throughout the solar cycle, including the current unusual solar minimum.

Chandra observations of comets 8p/Tuttle and 17p/Holmes during solar minimum

We present results for Chandra X-ray Observatory observations of two comets made during the minimum of solar cycle 24. The two comets, 17P/Holmes (17P) and 8P/Tuttle (8P), were very different in their activity and geometry. 17P was observed, for 30 ks right after its major outburst, on 2007 October 31 (10:07 UT), and comet 8P/Tuttle was observed in 2008 January for 47 ks. During the two Chandra observations, 17P was producing at least 100 times more water than 8P but was 2.2 times further away from the Sun. Also, 17P was at a relatively high solar latitude (+191) while 8P was observed at a lower solar latitude (34). The X-ray spectrum of 17P is unusually soft with little significant emission at energies above 500 eV. Depending on our choice of background, we derive a 300-1000 eV flux of 0.5-4.5 × 10–13 erg cm–2 s–1, with over 90% of the emission in the 300-400 eV range. This corresponds to an X-ray luminosity between 0.4 and 3.3 × 1015 erg s–1. However, we cannot distinguish between this significant excess emission and possible instrumental effects, such as incomplete charge transfer across the CCD. 17P is the first comet observed at high latitude during solar minimum. Its lack of X-rays in the 400-1000 eV range, in a simple picture, may be attributed to the polar solar wind, which is depleted in highly charged ions. 8P/Tuttle was much brighter, with an average count rate of 0.20 counts s–1 in the 300-1000 eV range. We derive an average X-ray flux in this range of 9.4 × 10–13 erg cm–2 s–1 and an X-ray luminosity for the comet of 1.7 × 1014 erg s–1. The light curve showed a dramatic decrease in flux of over 60% between observations on January 1 and 4. When comparing outer regions of the coma to inner regions, its spectra showed a decrease in ratios of C VI/C V, O VIII/O VII, as predicted by recent solar wind charge exchange (SWCX) emission models. There are remarkable differences between the X-ray emission from these two comets, further demonstrating the qualities of cometary X-ray observations, and SWCX emission in general as a means of remote diagnostics of the interaction of astrophysical plasmas.

Comparison of Heliospheric In Situ Data with the Quasi-steady Solar Wind Models

The standard theory for the solar-heliospheric magnetic field is the so-called quasi-steady model in which the field is determined by the observed magnetic flux at the photosphere and the balance between magnetic and plasma forces in the corona. In this model, the solar magnetic flux that opens to the heliosphere can increase or decrease as the photospheric flux evolves. The most sophisticated implementation of the quasi-steady theory is the SAIC model, which solves the fully time-dependent 3D MHD equations for the corona and wind until a steady state is achieved. In order to test the quasi-steady theory, we compare the 3D MHD model with observations of the heliospheric flux using multipoint measurements from the VHM instrument on the Ulysses spacecraft from 1991 to 2005 and from magnetic field measurements from various spacecraft at L1 compiled into the OMNI data set from 1976 through 2005. We also compare the observations to the predictions of the potential-field source-surface model, an older and simpler implementation of the quasi-steady theory. During solar maximum, ICMEs significantly disturb the heliospheric magnetic field, making our comparisons difficult. We find that the MHD model compares well with the general trends of the observed heliospheric fluxes. Variations on short timescales, presumably due to local effects, are missed by the model, but the long-term evolution is well matched. The model disagrees with observations most when Ulysses is in slow wind or ICME-related flows. The model underestimates the flux at solar maximum; however, this is to be expected, given the large number of ICMEs in the heliosphere at this time. We discuss the possible sources of discrepancy between the observations and the quasi-steady models.

A STATISTICAL STUDY OF THE AVERAGE IRON CHARGE STATE DISTRIBUTIONS INSIDE MAGNETIC CLOUDS FOR SOLAR CYCLE 23

Magnetic clouds (MCs) are the interplanetary counterpart of coronal magnetic flux ropes. They can provide valuable information to reveal the flux rope characteristics at their eruption stage in the corona, which are unable to be explored in situ at present. In this paper, we make a comprehensive survey of the average iron charge state ( Fe) distributions inside 96 MCs for solar cycle 23 using ACE (Advanced Composition Explorer) data. As the Fe in the solar wind are typically around 9+ to 11+, the Fe charge state is defined as high when the Fe is larger than 12+, which implies the existence of a considerable amount of Fe ions with high charge states (e.g., \geq 16+). The statistical results show that the Fe distributions of 92 (~ 96%) MCs can be classified into four groups with different characteristics. In group A (11 MCs), the Fe shows a bimodal distribution with both peaks higher than 12+. Group B (4 MCs) presents a unimodal distribution of Fe with its peak higher than 12+. In groups C (29 MCs) and D (48 MCs), the Fe remains higher and lower than 12+ throughout ACE passage through the MC, respectively. Possible explanations to these distributions are discussed.

OBSERVATIONS OF ENERGETIC PARTICLES BETWEEN A PAIR OF COROTATING INTERACTION REGIONS

We report observations of the acceleration and trapping of energetic ions and electrons between a pair of corotating interaction regions (CIRs). The event occurred in Carrington Rotation 2060. Observed by the STEREO-B spacecraft, the two CIRs were separated by less than 5 days. In contrast to other CIR events, the fluxes of the energetic ions and electrons in this event reached their maxima between the trailing edge of the first CIR and the leading edge of the second CIR. The radial magnetic field (Br ) reversed its sense and the anisotropy of the flux also changed from Sunward to anti-Sunward between the two CIRs. Furthermore, there was an extended period of counterstreaming suprathermal electrons between the two CIRs. Similar observations for this event were also obtained with the Advanced Composition Explorer and STEREO-A. We conjecture that these observations were due to a U-shaped, large-scale magnetic field topology connecting the reverse shock of the first CIR and the forward shock of the second CIR. Such a disconnected U-shaped magnetic field topology may have formed due to magnetic reconnection in the upper corona.

Case and statistical studies on the evolution of hot flow anomalies

Hot flow anomalies (HFAs) frequently observed near Earths bow shock are phenomena resulting from the interaction between interplanetary discontinuities and Earths bow shock. We identify 199 HFA events using Cluster data from 2001 to 2010 and divide these events into four categories according to the dynamic pressure profile, namely, “−+,” “+−,” “M,” and “W” types, where the symbols describe the profile of the dynamic pressure variations. We present case studies to show the main characteristics of each type of HFAs. Normalized superposed epoch analyses show that variations of the magnetic field magnitude of −+ and +− type HFAs are more dramatic than those of M and W types. The statistical study shows that the occurrence percentage of mature HFAs in W type HFAs is higher than that of −+, +−, and M types. Superposed epoch analysis result shows that variations of plasma parameters and magnetic field of mature HFAs are more dramatic than those of young HFAs, except for temperature. M and W type HFAs may be the later evolution stages of −+ and +− type HFAs; on the other hand, four categories of HFAs may be due to the fact that the spacecraft crossed an HFA structure along different paths.

Polar and equatorial coronal hole winds at solar minima: From the heliosphere to the inner corona

Fast solar wind can be accelerated from at least two different sources: polar coronal holes and equatorial coronal holes. Little is known about the relationship between the wind coming from these two different latitudes and whether these two subcategories of fast wind evolve in the same way during the solar cycle. Nineteen years of Ulysses observations, from 1990 to 2009, combined with ACE observations from 1998 to the present provide us with in situ measurements of solar wind properties that span two entire solar cycles. These missions provide an ideal data set to study the properties and evolution of the fast solar wind originating from equatorial and polar holes. In this work, we focus on these two types of fast solar wind during the minima between solar cycles 22 and 23 and 23 and 24. We use data from SWICS, SWOOPS, and VHM/FGM on board Ulysses and SWICS, SWEPAM, and MAG on board ACE to analyze the proton kinetic, thermal, and dynamic characteristics, heavy ion composition, and magnetic field properties of these two fast winds. The comparison shows that: (1) their kinetic, thermal, compositional, and magnetic properties are significantly different at any time during the two minima and (2) they respond differently to the changes in solar activity from cycle 23 to 24. These results indicate that equatorial and polar fast solar wind are two separate subcategories of fast wind. We discuss the implications of these results and relate them to remote-sensing measurements of the properties of polar and equatorial coronal holes carried out in the inner corona during these two solar minima.

INTERVALS OF RADIAL INTERPLANETARY MAGNETIC FIELDS AT 1 AU, THEIR ASSOCIATION WITH RAREFACTION REGIONS, AND THEIR APPARENT MAGNETIC FOOT POINTS AT THE SUN

We have examined 226 intervals of nearly radial interplanetary magnetic field orientations at 1 AU lasting in excess of 6 hr. They are found within rarefaction regions as are the previously reported high-latitude observations. We show that these rarefactions typically do not involve high-speed wind such as that seen by Ulysses at high latitudes during solar minimum. We have examined both the wind speeds and the thermal ion composition before, during and after the rarefaction in an effort to establish the source of the flow that leads to the formation of the rarefaction. We find that the bulk of the measurements, both fast- and slow-wind intervals, possess both wind speeds and thermal ion compositions that suggest they come from typical low-latitude sources that are nominally considered slow-wind sources. In other words, we find relatively little evidence of polar coronal hole sources even when we examine the faster wind ahead of the rarefaction regions. While this is in contrast to high-latitude observations, we argue that this is to be expected of low-latitude observations where polar coronal hole sources are less prevalent. As with the previous high-latitude observations, we contend that the best explanation for these periods of radial magnetic field is interchange reconnection between two sources of different wind speed.