H. A. Elliott

WEAKEST SOLAR WIND OF THE SPACE AGE AND THE CURRENT “MINI” SOLAR MAXIMUM

The last solar minimum, which extended into 2009, was especially deep and prolonged. Since then, sunspot activity has gone through a very small peak while the heliospheric current sheet achieved large tilt angles similar to prior solar maxima. The solar wind fluid properties and interplanetary magnetic field (IMF) have declined through the prolonged solar minimum and continued to be low through the current mini solar maximum. Compared to values typically observed from the mid-1970s through the mid-1990s, the following proton parameters are lower on average from 2009 through day 79 of 2013: solar wind speed and beta (~11%), temperature (~40%), thermal pressure (~55%), mass flux (~34%), momentum flux or dynamic pressure (~41%), energy flux (~48%), IMF magnitude (~31%), and radial component of the IMF (~38%). These results have important implications for the solar winds interaction with planetary magnetospheres and the heliospheres interaction with the local interstellar medium, with the proton dynamic pressure remaining near the lowest values observed in the space age: ~1.4 nPa, compared to ~2.4 nPa typically observed from the mid-1970s through the mid-1990s. The combination of lower magnetic flux emergence from the Sun (carried out in the solar wind as the IMF) and associated low power in the solar wind points to the causal relationship between them. Our results indicate that the low solar wind output is driven by an internal trend in the Sun that is longer than the ~11 yr solar cycle, and they suggest that this current weak solar maximum is driven by the same trend.

Solar Wind Influence on the Oxygen Content of Ion Outflow in the High Altitude Polar Cap During Solar Minimum Conditions

We correlate solar wind and interplanetary magnetic field (IMF) properties with the properties of O+ and H+ during early 1996 (solar minimum) at altitudes between 5.5 and 8.9 RE geocentric using the Thermal Ion Dynamics Experiment (TIDE) on the Polar satellite. Throughout the high-altitude polar cap we observe H+ to be more abundant than O+. O+ is found to be more abundant at lower latitudes when the solar wind speed is low (and Kp is low), while at higher solar wind speeds (and high Kp), O+ is observed across most of the polar cap. The O+ density and parallel flux are well organized by solar wind dynamic pressure, both increasing with solar wind dynamic pressure. Both the O+ density and parallel flux have positive correlations with both VswBIMF and Esw. No correlation is found between O+ density and IMF Bz, although a nonlinear relationship with IMF By is observed, possibly due to a strong linear correlation with the dynamic pressure. H+ is not as highly correlated with solar wind and IMF parameters, although H+ density and parallel flux are negatively correlated with IMF By and positively correlated with both VswBIMF and Esw. In this solar minimum data set, H+ is dominant, so that contributions of this plasma to the plasma sheet would have very low O+ to H+ ratios.

Current Sheet Evolution in the Aftermath of a CME Event

We report on SOHO UVCS observations of the coronal restructuring following a coronal mass ejection (CME) on 2002 November 26, at the time of a SOHO-Ulysses quadrature campaign. Starting about 1.5 hr after a CME in the northwest quadrant, UVCS began taking spectra at 1.7 R� , covering emission from both cool and hot plasma. Observations continued, with occasional gaps, for more than 2 days. Emission in the 974.8 8 line of [Fe xviii], indicating temperatures above 6 ; 10 6 K, was observed throughout the campaign in a spatially limited location. Comparison with EITimages shows the [Fe xviii] emission to overlie a growing post-flare loop system formed in the aftermath of the CME. The emission most likely originates in a current sheet overlying the arcade. Analysis of the [Fexviii] emission allows us to infer the evolution of physical parameters in the current sheet over the entire span of ourobservations:inparticular,wegivethetemperatureversustimeinthecurrentsheetandestimateitsdensity.Atthe timeofthequadrature,UlysseswasdirectlyabovethelocationoftheCMEandinterceptedtheejecta.Highionization state Fe was detected by the Ulysses SWICS throughout the magnetic cloud associated with the CME, although its rapid temporal variation suggests bursty, rather than smooth, reconnection in the coronal current sheet. The SOHOUlyssesdatasetprovideduswiththeuniqueopportunityofanalyzingacurrentsheetstructurefromitslowest coronal levels out to its in situ properties. Both the remote and in situ observations are compared with predictions of theoretical CME models.

The Solar Wind Around Pluto (SWAP) Instrument Aboard New Horizons

Abstract The Solar Wind Around Pluto (SWAP) instrument on New Horizons will measure the interaction between the solar wind and ions created by atmospheric loss from Pluto. These measurements provide a characterization of the total loss rate and allow us to examine the complex plasma interactions at Pluto for the first time. Constrained to fit within minimal resources, SWAP is optimized to make plasma-ion measurements at all rotation angles as the New Horizons spacecraft scans to image Pluto and Charon during the flyby. To meet these unique requirements, we combined a cylindrically symmetric retarding potential analyzer with small deflectors, a top-hat analyzer, and a redundant/coincidence detection scheme. This configuration allows for highly sensitive measurements and a controllable energy passband at all scan angles of the spacecraft.

Pluto's interaction with the solar wind

This study provides the first observations of Plutogenic ions and their unique interaction with the solar wind. We find ~20% solar wind slowing that maps to a point only ~4.5 RP upstream of Pluto and a bow shock most likely produced by comet-like mass loading. The Pluto obstacle is a region of dense heavy ions bounded by a “Plutopause” where the solar wind is largely excluded and which extends back >100 RP into a heavy ion tail. The upstream standoff distance is at only ~2.5 RP. The heavy ion tail contains considerable structure, may still be partially threaded by the interplanetary magnetic field (IMF), and is surrounded by a light ion sheath. The heavy ions (presumably CH4+) have average speed, density, and temperature of ~90 km s−1, ~0.009 cm−3, and ~7 × 105 K, with significant variability, slightly increasing speed/temperature with distance, and are N-S asymmetric. Density and temperature are roughly anticorrelated yielding a pressure ~2 × 10−2 pPa, roughly in balance with the interstellar pickup ions at ~33 AU. We set an upper bound of <30 nT surface field at Pluto and argue that the obstacle is largely produced by atmospheric thermal pressure like Venus and Mars; we also show that the loss rate down the tail (~5 × 1023 s−1) is only ~1% of the expected total CH4 loss rate from Pluto. Finally, we observe a burst of heavy ions upstream from the bow shock as they are becoming picked up and tentatively identify an IMF outward sector at the time of the NH flyby.

An improved expected temperature formula for identifying interplanetary coronal mass ejections

[1] In this study we compare nearly 5 years of solar wind proton speed and temperature measurements from the Solar Wind Electron Proton Alpha Monitor (SWEPAM) on the Advanced Composition Explorer (ACE) to derive an improved expected temperature formula to identify interplanetary coronal mass ejections (ICMEs). Anomalously low proton temperatures have long been associated with ICMEs. When transient ICMEs are not present, the solar wind speed and temperature are highly correlated, and previous studies have derived fits to these measurements. Using these fits, an expected temperature is determined from the solar wind speed. Anomalously low temperatures have been identified as times when the ratio of the measured to expected temperature is below 0.5. In this study we remove ICMEs before fitting the remaining data. Fast and slow parcels in the solar wind interact and cause compressions and rarefactions as the solar wind moves away from the Sun. Since such interaction causes the speed and temperature of these parcels to change, we separately fit compression and rarefactions. We find that the expected temperature formula derived in this way provides a better way of identifying ICMEs than previous formulas, particularly in compression regions.

CME Propagation Characteristics from Radio Observations

Abstract We explore the relationship among three coronal mass ejections (CMEs), observed on 28 October 2003, 7 November 2004, and 20 January 2005, the type II burst-associated shock waves in the corona and solar wind, as well as the arrival of their related shock waves and magnetic clouds at 1 AU. Using six different coronal/interplanetary density models, we calculate the speeds of shocks from the frequency drifts observed in metric and decametric radio wave data. We compare these speeds with the velocity of the CMEs as observed in the plane-of-the-sky white-light observations and calculated with a cone model for the 7 November 2004 event. We then follow the propagation of the ejecta using Interplanetary Scintillation measurements, which were available for the 7 November 2004 and 20 January 2005 events. Finally, we calculate the travel time of the interplanetary shocks between the Sun and Earth and discuss the velocities obtained from the different data. This study highlights the difficulties in making velocity estimates that cover the full CME propagation time.

Pluto’s interaction with its space environment: Solar wind, energetic particles, and dust

New Horizons unveils the Pluto system In July 2015, the New Horizons spacecraft flew through the Pluto system at high speed, humanitys first close look at this enigmatic system on the outskirts of our solar system. In a series of papers, the New Horizons team present their analysis of the encounter data downloaded so far: Moore et al. present the complex surface features and geology of Pluto and its large moon Charon, including evidence of tectonics, glacial flow, and possible cryovolcanoes. Grundy et al. analyzed the colors and chemical compositions of their surfaces, with ices of H2O, CH4, CO, N2, and NH3 and a reddish material which may be tholins. Gladstone et al. investigated the atmosphere of Pluto, which is colder and more compact than expected and hosts numerous extensive layers of haze. Weaver et al. examined the small moons Styx, Nix, Kerberos, and Hydra, which are irregularly shaped, fast-rotating, and have bright surfaces. Bagenal et al. report how Pluto modifies its space environment, including interactions with the solar wind and a lack of dust in the system. Together, these findings massively increase our understanding of the bodies in the outer solar system. They will underpin the analysis of New Horizons data, which will continue for years to come. Science, this issue pp. 1284, 10.1126/science.aad9189, 10.1126/science.aad8866, 10.1126/science.aae0030, & 10.1126/science.aad9045 Pluto modifies its space environment, interacting with the solar wind plasma and energetic particles. INTRODUCTION The scientific objectives of NASA’s New Horizons mission include quantifying the rate at which atmospheric gases are escaping Pluto and describing its interaction with the surrounding space environment. The two New Horizons instruments that measure charged particles are the Solar Wind Around Pluto (SWAP) instrument and the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instrument. The Venetia Burney Student Dust Counter (SDC) counts the micrometer-sized dust grains that hit the detectors mounted on the ram direction of the spacecraft. This paper describes preliminary results from these three instruments when New Horizons flew past Pluto in July 2015 at a distance of 32.9 astronomical units (AU) from the Sun. RATIONALE Initial studies of the solar wind interaction with Pluto’s atmosphere suggested that the extent of the interaction depends on whether the atmospheric escape flux is strong (producing a comet-like interaction, where the interaction region is dominated by ion pick-up and is many times larger than the object) or weak (producing a Mars-like interaction dominated by ionospheric currents with limited upstream pick-up and where the scale size is comparable to the object). Before the New Horizons flyby, the estimates of the atmospheric escape rate ranged from as low as 1.5 × 1025 molecules s–1 to as high as 2 × 1028 molecules s–1. Combining these wide-ranging predictions of atmospheric escape rates with Voyager and New Horizons observations of extensive variability of the solar wind at 33 AU produced estimates of the scale of the interaction region that spanned all the way from 7 to 1000 Pluto radii (RP). RESULTS At the time of the flyby, SWAP measured the solar wind conditions near Pluto to be nearly constant and stronger than usual. The abnormally high solar wind density and associated pressures for this distance are likely due to a relatively strong traveling interplanetary shock that passed over the spacecraft 5 days earlier. Heavy ions picked up sunward from Pluto should mass-load and slow the solar wind. However, there is no evidence of such solar wind slowing in the SWAP data taken as near as ~20 RP inbound, which suggests that very few atmospheric molecules are escaping upstream and becoming ionized. The reorientation of the spacecraft to enable imaging of the Pluto system meant that both the SWAP and PEPSSI instruments were turned away from the solar direction, thus complicating our analysis of the particle data. Nevertheless, when the spacecraft was ~10 RP from Pluto, SWAP data indicated that the solar wind had slowed by ~20%. We use these measurements to estimate a distance of ~6 RP for the 20% slowing location directly upstream of Pluto. At this time, PEPSSI detected an enhancement of ions with energies in the kilo–electron volt range. The SDC, which measures grains with radii >1.4 µm, detected one candidate impact in ±5 days around its closest approach, indicating a dust density estimate of n = 1.2 km–3, with a 90% confidence level range of 0.6 < n < 4.6 km–3. CONCLUSION New Horizons’s particle instruments revealed an interaction region confined sunward of Pluto to within ~6 RP. The surprisingly small size is consistent with a reduced atmospheric escape rate of 6 × 1025 CH4 molecules s–1, as well as a particularly high solar wind flux due to a passing compression region. This region is similar in scale to the solar wind interaction with Mars’s escaping atmosphere. Beyond Pluto, the disturbance persists to distances greater than 400 RP downstream. Interaction of the solar wind with Pluto’s extended atmosphere. Protons and electrons streaming from the Sun at ~400 km s–1 are slowed and deflected around Pluto because of a combination of ionization of Pluto’s atmosphere and electrical currents induced in Pluto’s ionosphere. CREDIT: STEVE BARTLETT AND NASA’S SCIENTIFIC VISUALIZATION STUDIO The New Horizons spacecraft carried three instruments that measured the space environment near Pluto as it flew by on 14 July 2015. The Solar Wind Around Pluto (SWAP) instrument revealed an interaction region confined sunward of Pluto to within about 6 Pluto radii. The region’s surprisingly small size is consistent with a reduced atmospheric escape rate, as well as a particularly high solar wind flux. Observations from the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instrument suggest that ions are accelerated and/or deflected around Pluto. In the wake of the interaction region, PEPSSI observed suprathermal particle fluxes equal to about 1/10 of the flux in the interplanetary medium and increasing with distance downstream. The Venetia Burney Student Dust Counter, which measures grains with radii larger than 1.4 micrometers, detected one candidate impact in ±5 days around New Horizons’ closest approach, indicating an upper limit of <4.6 kilometers–3 for the dust density in the Pluto system.

ENERGETIC PARTICLE OBSERVATIONS AND PROPAGATION IN THE THREE-DIMENSIONAL HELIOSPHERE DURING THE 2006 DECEMBER EVENTS

We report observations of solar energetic particles obtained by the HI-SCALE and COSPIN/LET instruments onboard Ulysses during the period of isolated but intense solar activity in 2006 December, in the declining phase of the solar activity cycle. We present measurements of particle intensities and also discuss observations of particle anisotropies and composition in selected energy ranges. Active Region 10930 produced a series of major solar flares with the strongest one (X9.0) recorded on December 5 after it rotated into view on the solar east limb. Located over the South Pole of the Sun, at >72°S heliographic latitude and 2.8 AU radial distance, Ulysses provided unique measurements for assessing the nature of particle propagation to high latitudes under near-minimum solar activity conditions, in a relatively undisturbed heliosphere. The observations seem to exclude the possibility that magnetic field lines originating at low latitudes reached Ulysses, suggesting either that the energetic particles observed as large solar energetic particle (SEP) events over the South Pole of the Sun in 2006 December were released when propagating coronal waves reached high-latitude field lines connected to Ulysses, or underwent perpendicular diffusion. We also discuss comparisons with energetic particle data acquired by the STEREO and Advanced Composition Explorer in the ecliptic plane near 1 AU during this period.

COMETARY IONS TRAPPED IN A CORONAL MASS EJECTION

Ion tails of comets are known to extend radially away from the Sun over very large distances. Crossing these tails by spacecraft not specifically targeted to intercept them was believed to be extremely improbable, since that requires precise angular alignment of the spacecraft with a comet. We report here the fortuitous detection of cometary ions at large angular separation far from the comet. To explain this unexpected discovery, we conclude that these ions were ducted laterally along magnetic fields that were randomly distorted by a coronal mass ejection and that such transport increases the probability of an unplanned detection of comets.