near-Earth Measurements Research Articles

On the Production of He+ of Solar Origin in the Solar Wind

Abstract Solar wind measurements in the heliosphere predominantly comprise protons, alphas, and minor elements in a highly ionized state. The majority of low-charge states, such as He+, measured in situ are often attributed to pick-up ions of nonsolar origin. However, through inspection of the velocity distribution functions of near-Earth measurements, we find a small but significant population of He+ ions in the normal solar wind whose properties indicate that it originated from the Sun and has evolved as part of the normal solar wind. Current ionization models, largely governed by electron impact and radiative ionization and recombination processes, underestimate this population by several orders of magnitude. Therefore, to reconcile the singly ionized He observed, we investigate the recombination of solar He2+ through charge exchange with neutrals from circumsolar dust as a possible formation mechanism of solar He+. We present an empirical profile of neutrals necessary for charge exchange to become an effective vehicle to recombine He2+ to He+ such that it meets observational He+ values. We find that the formation of He+ is not only sensitive to the density of neutrals but also to the inner boundary of the neutral distribution encountered along the solar wind path. However, further observational constraints are necessary to confirm that the interaction between solar α particles and dust neutrals is the primary source of the He+ observations.

Open-field Coronal Structures Neighbouring the Sunspot of AR 8535

We make use of the Potential Field Source Surface (PFSS) model and reveal new details in the strong fields of the dominant sunspot of the active region (AR) 8535. Due to its importance in determining the field line connectivity and topology with resulting consequences for both plasma confinement and dynamics we construct the global magnetic fields with the PFSS model. The coronal emission – observed in the soft X-ray images taken with the Yohkoh Soft X-ray Telescope and 284 A images taken with the Extreme-ultraviolet Imaging Telescope aboard the Solar and HeliosphericObservatory (SOHO/EIT) – is largely asymmetric owing to two dark coronal lanes, which are traceable across the sunspot. The modelled open-field structures overlap with the observed coronal dark lanes and include two narrow coronal corridors that persist for days in the large sunspot. Since the open-field structures overlap also with the plasma upflow observed in the sunspot as Doppler blue-shifts in a set of EUV emission lines (SOHO/Coronal Diagnostic Spectrometer) on 9 May and 13 May 1999, we examine the plasma outflow from the AR 8535. From near-Earth measurements aboard the Advanced Composition Explorer (ACE) spacecraft we estimate the solar-wind speed $V _{\mathrm{p}} \approx 400~\mbox{km}\,\mbox{s}^{-1}$ and the charge state ratio $\mbox{O}^{7+}/\mbox{O}^{6+} = 0.4$ (corresponding to coronal “freezing-in temperature” $T _{\mathrm{e}} = 1.7 \times 10^{6}~\mbox{K}$). However, the abnormally low proton temperature of solar wind, $T _{\mathrm{p}} = 3 \times 10^{4}~\mbox{K}$, implies some impact of the interplanetary coronal mass ejection (ICME) on the plasma flow. The evidence in favour of the outflow from AR 8535 is given by the semi-empirical Wang–Sheeley–Arge model, as it predicts the ACE measurements of solar wind and the relevant features in the tomographic velocity map resulted from the Solar-Terrestrial Environment Laboratory observations of the interplanetary scintillations. Finally, we propose constructing a model atmosphere of the sunspot with the narrow coronal corridors included in order to further elaborate the magnetic-field structure through its effect on the observed microwave radiation.

Solar Angular Momentum Loss over the Past Several Millennia

Abstract The Sun and Sun-like stars lose angular momentum to their magnetized stellar winds. This braking torque is coupled to the stellar magnetic field, such that changes in the strength and/or geometry of the field modifies the efficiency of this process. Since the space age, we have been able to directly measure solar wind properties using in situ spacecraft. Furthermore, indirect proxies such as sunspot number, geomagnetic indices, and cosmogenic radionuclides, constrain the variation of solar wind properties on centennial and millennial timescales. We use near-Earth measurements of the solar wind plasma and magnetic field to calculate the torque on the Sun throughout the space age. Then, reconstructions of the solar open magnetic flux are used to estimate the time-varying braking torque during the last nine millennia. We assume a relationship for the solar mass-loss rate based on observations during the space age which, due to the weak dependence of the torque on mass-loss rate, does not strongly affect our predicted torque. The average torque during the last nine millennia is found to be 2.2 × 1030 erg, which is comparable to the average value from the last two decades. Our data set includes grand minima (such as the Maunder Minimum), and maxima in solar activity, where the torque varies from ∼1 to 5 × 1030 erg (averaged on decadal timescales), respectively. We find no evidence for any secular variation of the torque on timescales of less than 9000 yr.

Approaching Solar Maximum 24 with STEREO—Multipoint Observations of Solar Energetic Particle Events

Since the beginning of the Solar Terrestrial Relations Observatory (STEREO) mission at the end of 2006, the two spacecraft have now separated by more than 130 ∘ degrees from the Earth. A 360-degree view of the Sun has been possible since February 2011, providing multipoint in situ and remote sensing observations of unprecedented quality. Combining STEREO observations with near-Earth measurements allows the study of solar energetic particle (SEP) events over a wide longitudinal range with minimal radial gradient effects. This contribution provides an overview of recent results obtained by the STEREO/IMPACT team in combination with observations by the ACE and SOHO spacecraft. We focus especially on multi-spacecraft investigations of SEP events. The large longitudinal spread of electron and 3He-rich events as well as unusual anisotropies will be presented and discussed.

Modeling the Sun's open magnetic flux

Context. The heliospheric magnetic field can be extrapolated from the photospheric field distribution using models based upon various approximations. Such models are required for the reconstruction of the open solar magnetic flux prior to the time of direct measurements on the basis of surface flux transport simulations. Aims. We evaluate the consistency of extrapolation models with direct measurements of the heliospheric magnetic field. Furthermore, we study whether extrapolations on the basis of a surface flux transport model for the photospheric magnetic field reproduce the temporal evolution of the measured near-Earth magnetic field. Methods. We use the potential field source surface (PFSS) model and the current sheet source surface (CSSS) model to extrapolate the heliospheric field on the basis of the Wilcox Solar Observatory (WSO) synoptic maps of the solar surface field from 1976-2005. The results are compared with the near-Earth measurements of the radial heliospheric field and its independence of latitude found with Ulysses. Furthermore, we determine extrapolations on the basis of photospheric flux distributions computed with a surface flux transport code, using as input sunspot group areas from the SOON database. Results. The CSSS model based upon WSO data, with a source surface located at >10 R ⊙ and cusp surface at 1.7 R ⊙ . yields the best agreement with the measurements. The flux transport simulations reproduce the observed surface flux together with the open flux if the tilt angle of emerging bipolar magnetic regions is smaller than commonly assumed, but consistent with sunspot observations. Conclusions. The CSSS model with a source surface in the vicinity of the Alfvenic point of the solar wind leads to a good extrapolation of the heliospheric field from solar surface data. A surface flux transport model based upon sunspot data with consistently calibrated tilt angles reproduces the observed evolution of the solar total open flux.

Open solar flux estimates from near-Earth measurements of the interplanetary magnetic field: comparison of the first two perihelion passes of the Ulysses spacecraft

Abstract. Results from all phases of the orbits of the Ulysses spacecraft have shown that the magnitude of the radial component of the heliospheric field is approximately independent of heliographic latitude. This result allows the use of near-Earth observations to compute the total open flux of the Sun. For example, using satellite observations of the interplanetary magnetic field, the average open solar flux was shown to have risen by 29% between 1963 and 1987 and using the aa geomagnetic index it was found to have doubled during the 20th century. It is therefore important to assess fully the accuracy of the result and to check that it applies to all phases of the solar cycle. The first perihelion pass of the Ulysses spacecraft was close to sunspot minimum, and recent data from the second perihelion pass show that the result also holds at solar maximum. The high level of correlation between the open flux derived from the various methods strongly supports the Ulysses discovery that the radial field component is independent of latitude. We show here that the errors introduced into open solar flux estimates by assuming that the heliospheric field's radial component is independent of latitude are similar for the two passes and are of order 25% for daily values, falling to 5% for averaging timescales of 27 days or greater. We compare here the results of four methods for estimating the open solar flux with results from the first and second perehelion passes by Ulysses. We find that the errors are lowest (1–5% for averages over the entire perehelion passes lasting near 320 days), for near-Earth methods, based on either interplanetary magnetic field observations or the aa geomagnetic activity index. The corresponding errors for the Solanki et al. (2000) model are of the order of 9–15% and for the PFSS method, based on solar magnetograms, are of the order of 13–47%. The model of Solanki et al. is based on the continuity equation of open flux, and uses the sunspot number to quantify the rate of open flux emergence. It predicts that the average open solar flux has been decreasing since 1987, as is observed in the variation of all the estimates of the open flux. This decline combines with the solar cycle variation to produce an open flux during the second (sunspot maximum) perihelion pass of Ulysses which is only slightly larger than that during the first (sunspot minimum) perihelion pass. Key words. Interplanetary physics (interplanetary magnetic fields) – Solar physics, astrophysics and astronomy (magnetic fields)

An examination of possible solar wind sources for a sudden brightening of comet IRAS‐Araki‐Alcock

Possible solar wind sources for a sudden global brightening of Comet IRAS‐Araki‐Alcock are examined. No increases in solar wind momentum flux, solar energetic particles or solar activity occurred coincident with these brightenings. The only change in the solar wind coincident with the brightenings was a rotation of the interplanetary magnetic field to a more flow‐aligned state. If this rotation did not lead to the cometary brightening, the brightening must have been intrinsic to the comet.