DOCSLIB.ORG

Lecture 17: the Solar Wind O Topics to Be Covered

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Lecture 17: the Solar Wind O Topics to Be Covered Lecture 17: The solar wind o Topics to be covered: o Solar wind o Inteplanetary magnetic field The solar wind o Biermann (1951) noticed that many comets showed excess ionization and abrupt changes in the outflow of material in their tails - is this due to a solar wind? o Assumed comet orbit perpendicular to line-of-sight (vperp) and tail at angle ! => tan! = vperp/vr o From observations, tan ! ~ 0.074 o But vperp is a projection of vorbit -1 => vperp = vorbit sin ! ~ 33 km s -1 o From 600 comets, vr ~ 450 km s . o See Uni. New Hampshire course (Physics 954) for further details: http://www-ssg.sr.unh.edu/Physics954/Syllabus.html The solar wind o STEREO satellite image sequences of comet tail buffeting and disconnection. Parker’s solar wind o Parker (1958) assumed that the outflow from the Sun is steady, spherically symmetric and isothermal. o As PSun>>PISM => must drive a flow. o Chapman (1957) considered corona to be in hydrostatic equibrium: dP = −ρg dr dP GMS ρ + 2 = 0 Eqn. 1 dr r o If first term >> than second €=> produces an outflow: € dP GM ρ dv + S + ρ = 0 Eqn. 2 dr r2 dt o This is the equation for a steadily expanding solar/stellar wind. € Parker’s solar wind (cont.) dv dv dr dv dP GM ρ dv o As, = = v => + S + ρv = 0 dt dr dt dr dr r2 dr dv 1 dP GM or v + + S = 0 Eqn. 3 dr ρ dr r2 € € o Called the momentum equation. € o Eqn. 3 describes acceleration (1st term) of the gas due to a pressure gradient (2nd term) and gravity (3rd term). Need Eqn. 3 in terms of v. o Assuming a perfect gas, P = R ! T / ! (R is gas constant; ! is mean atomic weight), the 2nd term of Eqn. 3 is: dP Rρ dT RT dρ = + dr µ dr µ dr Isothermal wind => dT/dr 0 1 dP $ RT ' 1 dρ ! ⇒ = & ) Eqn. 4 ρ dr % µ ( ρ dr € Parker’s solar wind (cont.) o Now, the mass loss rate is assumed to be constant, so the Equation of Mass Conservation is: dM = 4πr2ρv = const ⇒ r2ρv = const Eqn. 5 dt d(r2ρv) dv dr2 dρ o Differentiating, = r2ρ + ρv + r2v = 0 dr dr dr dr 1 dρ 1 dv 2 € => = − − Eqn. 6 ρ dr v dr r o Substituting Eqn. 6€ into Eqn. 4, and into the 2nd term of Eqn. 3, we get dv RT # 1 dv 2& GM € v + % − − ( + S = 0 dr µ $ v dr r' r2 # & RT dv 2RT GMS ⇒ % v − ( − + 2 = 0 $ µv ' dr µr r 2RT GM o A critical point occurs when dv/dr ! 0 i.e., when = S µr r2 € 2 o Setting vc = RT /µ => rc = GMS /2vc € € Parker’s solar wind (cont.) 1 dv v 2 o Rearranging => v 2 − v 2 = 2 c (r − r ) Eqn. 7 ( c ) v dr r2 c o Gives the momentum equation in terms of the flow velocity. € o If r = rc, dv/dr -> 0 or v = vc, and if v = vc, dv/dr -> ∞ or r = rc. o An acceptable solution is when r = rc and v = vc (critical point). o A solution to Eqn. 7 can be found by direct integration: 2 2 " v % " v % " r % r Eqn. 8 $ ' − ln$ ' = 4ln$ ' + 4 c + C Parker’s “Solar # vc & # vc & # rc & r Wind Solutions” where C is a constant of integration. Leads to five solutions depending on C. € Parker’s solutions o Solution I and II are double valued. Solution II also doesn’t connect to the solar surface. v/vc o Solution III is too large (supersonic) close to the Sun - not observed. Critical point o Solution IV is called the solar breeze solution. o Solution V is the solar wind solution (confirmed r/r in 1960 by Mariner II). It c passes through the critical point at r = rc and v = vc. Parker’s solutions (cont.) o Look at Solutions IV and V in more detail. o Solution IV: For large r, v! 0 and Eqn. 8 reduces to: 2 2 # v & # r & v # r & −ln% ( ≈ 4ln% ( ⇒ = % ( $ vc ' $ rc ' vc $ rc ' 2 2 1 o Therefore, r v ! rc vc = const or v ≈ r2 € const const o From Eqn. 5: ρ = 2 = 2 = const r v rc vc € o From Ideal Gas Law: P∞ = R ! ∞ T / ! => P∞ = const € o The solar breeze solution results in high density and pressure at large r =>unphysical solution. Parker’s solutions (cont.) o Solution V: From the figure, v >> vc for large r. Eqn. 8 can be written: 2 " v % " r % " r % $ ' ≈ 4ln$ ' ⇒ v ≈ vc 2 ln$ ' # vc & # rc & # rc & const const o The density is then: ρ = ≈ r2v r2 ln(r /r ) € c => ! ! 0 as r ! ∞. o As plasma is isothermal€ (i.e., T = const.), Ideal Gas Law => P ! 0 as r !∞. o This solution eventually matches interstellar gas properties => physically realistic model. o Solution V is called the solar wind solution. Observed solar wind o Fast solar wind (>500 km s-1) comes from coronal holes. o Slow solar wind (<500 km s-1) comes from closed magnetic field areas. o Figure from McComas et al., Geophysical Research Letters, (2008). Interplanetary magnetic field B! or v! ! Br or vr o Solar rotation drags magnetic field into an Archimedian spiral (r = a!). o Predicted by Eugene Parker => Parker B Spiral: r r - r0 = -(v/!)(! - !0) o Winding angle: Bφ vφ (r , ! ) tanψ = = 0 0 Br vr Ω(r − r ) ! = 0 vr o Inclined at ~45º at 1 AU ~90º by 10 AU. € Alfven radius o Close to the Sun, the solar wind is too weak to modify structure of magnetic field: B2 1/2ρv 2 << 8π o Solar magnetic field therefore forces the solar wind to co-rotate with the Sun. o When the solar wind becomes super-Alfvenic B2 € 1/2ρv 2 >> 8π o This typically occurs at ~50 Rsun (0.25 AU). B2 o Transition between regimes occurs at the Alfven radius (r ), where 1/2ρv 2 = € A 8π M o Assuming the Sun’s field to be a dipole, B = 3 r € 1/ 6 $ M 2 ' => rA = & ) 4πρv 2 € % ( € The Parker spiral Heliosphere .
Load more

Recommended publications
  • Observations of Solar Wind Penetration Into the Earth's Magnetosphere: the Plasma Mantle
    ENNIO R. SANCHEZ, CHING-I. MENG, and PATRICK T. NEWELL OBSERVATIONS OF SOLAR WIND PENETRATION INTO THE EARTH'S MAGNETOSPHERE: THE PLASMA MANTLE The large database provided by the continuous coverage of the Defense Meteorological Satellite Pro­ gram polar orbiting satellites constitutes an important source of information on particle precipitation in the ionosphere. This information can be used to monitor and map the Earth's magnetosphere (the cavity around the Earth that forms as the stream of particles and magnetic field ejected from the Sun, known as the solar wind, encounters the Earth's magnetic field) and for a large variety of statistical studies of its morphology and dynamics. The boundary between the magnetosphere and the solar wind is pre­ sumably open in some places and at some times, thus allowing the direct entry of solar-wind plasma into the magnetosphere through a boundary layer known as the plasma mantle. The preliminary results of a statistical study of the plasma-mantle precipitation in the ionosphere are presented. The first quan­ titative mapping of the ionospheric region where the plasma-mantle particles precipitate is obtained. INTRODUCTION Polar orbiting satellites are very useful platforms for studying the properties of the environment surrounding the Earth at distances well above the ionosphere. This article focuses on a description of the enormous poten­ tial of those platforms, especially when they are com­ bined with other means of measurement, such as ground-based stations and other satellites. We describe in some detail the first results of the kind of study for which the polar orbiting satellites are ideal instruments.
    [Show full text]
  • Messenger Extreme Exploration
    MESSENGERMESSENGER EEXTREMEXTREME EEXPLORATIONXPLORATION — Deborah Domingue, MESSENGER Deputy Project Scientist, Johns Hopkins University Applied Physics Laboratory The first half of 2004 has been an exciting time for planetary exploration with the successful encounter and sampling of Comet Wild 2 by Stardust, the landing of two Mars rovers, orbital operations at Mars by three spacecraft, and the arrival of Cassini at Saturn. This month we wished “Bon Voyage” to the MESSENGER spacecraft, which successfully launched from Cape Canaveral on August 3, 2004, and began its journey to the innermost planet of our solar system, Mercury. MESSENGER is a MEcury Surface, Space ENvironment, GEochemistry, and Ranging mission that will orbit Mercury for one Earth year (beginning in March 2011) following three flybys (in January and Septem- ber 2008 and October 2009) of the planet. It will be our first return to Mercury in over 30 years! The Mariner 10 spacecraft flew past Mer- cury three times in 1974 and 1975 and collected information on less than half the planet. MESSENGER will provide the first global map of Mercury, in addition to detailed information on the composition and structure of Mercury’s crust, its geologic history, the nature of Mercury’s thin exosphere and dynamic magnetosphere, and the makeup of its core and polar materials. MERCURY:A PLACE OF EXTREMES Mercury, the planet closest to the Sun, has a highly elliptical orbit such that its distance from the Sun ranges from 46 million kilometers (29 million miles) to 70 million kilometers (43 million miles). Because of its slow rotation, Mercury’s day (sunrise to sunrise) actually lasts two Mercury years (88 Earth days of dark and 88 Earth days of daylight).
    [Show full text]
  • THEMIS Telescope Images Analysed for Space Weather Traces
    EPSC Abstracts Vol. 14, EPSC2020-1022, 2020 https://doi.org/10.5194/epsc2020-1022 Europlanet Science Congress 2020 © Author(s) 2021. This work is distributed under the Creative Commons Attribution 4.0 License. THEMIS telescope images analysed for space weather traces Melinda Dósa1, Valeria Mangano2, Zsofia Bebesi1, Stefano Massetti2, Anna Milillo2, and Anna Görgei3 1Wigner Research Centre for Physics, Space Physics and Space Technology, Budapest, Hungary ([email protected]) 2INAF/IAPS, Istituto Nazionale di Astrofisica, Roma, Italy 3Eötvös Loránd University, Institute of Physics The THEMIS solar telescope operating on Tenerife (Canary islands) has observed Mercury’s Na exosphere along several campaigns since 2007. A dataset of images taken between 2009 and 2013 are analysed here in relation with propagated solar wind data. A small subset of the images shows a low level of correlation between Na-emission and solar wind dynamic pressure. The amount of data at present is not sufficient to make a clear statement on whether the correlation is a coincidence or can be explained by other factors (position of Mercury and Earth, solar activity, etc.). Nevertheless, the authors present a comprehensive study taking into account all possible factors. Sodium plays a special role in Mercury’s exosphere: due to its strong resonance line it has been observed and monitored by Earth-based telescopes for decades. Different and highly variable patterns of Na-emission have been identified, the most common and recurrent being the high latitude double-peak pattern [1]. It is clear that the exosphere is linked to the surface and influenced by the interstellar medium and the solar wind deviated by the magnetosphere, but the role and weight of the single processes are still under discussion [2].
    [Show full text]
  • Cmes, Solar Wind and Sun-Earth Connections: Unresolved Issues
    CMEs, solar wind and Sun-Earth connections: unresolved issues Rainer Schwenn Max-Planck-Institut für Sonnensystemforschung, Katlenburg-Lindau, Germany [email protected] In recent years, an unprecedented amount of high-quality data from various spaceprobes (Yohkoh, WIND, SOHO, ACE, TRACE, Ulysses) has been piled up that exhibit the enormous variety of CME properties and their effects on the whole heliosphere. Journals and books abound with new findings on this most exciting subject. However, major problems could still not be solved. In this Reporter Talk I will try to describe these unresolved issues in context with our present knowledge. My very personal Catalog of ignorance, Updated version (see SW8) IAGA Scientific Assembly in Toulouse, 18-29 July 2005 MPRS seminar on January 18, 2006 The definition of a CME "We define a coronal mass ejection (CME) to be an observable change in coronal structure that occurs on a time scale of a few minutes and several hours and involves the appearance (and outward motion, RS) of a new, discrete, bright, white-light feature in the coronagraph field of view." (Hundhausen et al., 1984, similar to the definition of "mass ejection events" by Munro et al., 1979). CME: coronal -------- mass ejection, not: coronal mass -------- ejection! In particular, a CME is NOT an Ejección de Masa Coronal (EMC), Ejectie de Maså Coronalå, Eiezione di Massa Coronale Éjection de Masse Coronale The community has chosen to keep the name “CME”, although the more precise term “solar mass ejection” appears to be more appropriate. An ICME is the interplanetry counterpart of a CME 1 1.
    [Show full text]
  • A Brief History of Magnetospheric Physics Before the Spaceflight Era
    A BRIEF HISTORY OF MAGNETOSPHERIC PHYSICS BEFORE THE SPACEFLIGHT ERA David P. Stern Laboratoryfor ExtraterrestrialPhysics NASAGoddard Space Flight Center Greenbelt,Maryland Abstract.This review traces early resea/ch on the Earth's aurora, plasma cloud particles required some way of magneticenvironment, covering the period when only penetratingthe "Chapman-Ferrarocavity": Alfv•n (1939) ground:based0bservationswerepossible. Observations of invoked an eleCtric field, but his ideas met resistance. The magneticstorms (1724) and of perturbationsassociated picture grew more complicated with observationsof with the aurora (1741) suggestedthat those phenomena comets(1943, 1951) which suggesteda fast "solarwind" originatedoutside the Earth; correlationof the solarcycle emanatingfrom the Sun's coronaat all times. This flow (1851)with magnetic activity (1852) pointed to theSun's was explainedby Parker's theory (1958), and the perma- involvement.The discovei-yof •solarflares (1859) and nent cavity which it producedaround the Earth was later growingevidence for their associationwith large storms named the "magnetosphere"(1959). As early as 1905, led Birkeland (1900) to proposesolar electronstreams as Birkeland had proposedthat the large magneticperturba- thecause. Though laboratory experiments provided some tions of the polar aurora refleCteda "polar" type of support;the idea ran into theoreticaldifficulties and was magneticstorm whose electric currents descended into the replacedby Chapmanand Ferraro's notion of solarplasma upper atmosphere;that idea, however, was resisted for clouds (1930). Magnetic storms were first attributed more than 50 years. By the time of the International (1911)to a "ringcurrent" of high-energyparticles circling GeophysicalYear (1957-1958), when the first artificial the Earth, but later work (1957) reCOgnizedthat low- satelliteswere launched, most of the importantfeatures of energy particlesundergoing guiding center drifts could the magnetospherehad been glimpsed, but detailed have the same effect.
    [Show full text]
  • Solar Wind Magnetosphere Coupling
    Solar Wind Magnetosphere Coupling F. Toffoletto, Rice University Figure courtesy T. W. Hill with thanks to R. A. Wolf and T. W. Hill, Rice U. Outline • Introduction • Properties of the Solar Wind Near Earth • The Magnetosheath • The Magnetopause • Basic Physical Processes that control Solar Wind Magnetosphere Coupling – Open and Closed Magnetosphere Processes – Electrodynamic coupling – Mass, Momentum and Energy coupling – The role of the ionosphere • Current Status and Summary QuickTime™ and a YUV420 codec decompressor are needed to see this picture. Introduction • By virtue of our proximity, the Earth’s magnetosphere is the most studied and perhaps best understood magnetosphere – The system is rather complex in its structure and behavior and there are still some basic unresolved questions – Today’s lecture will focus on describing the coupling to the major driver of the magnetosphere - the solar wind, and the ionosphere – Monday’s lecture will look more at the more dynamic (and controversial) aspect of magnetospheric dynamics: storms and substorms The Solar Wind Near the Earth Solar-Wind Properties Observed Near Earth • Solar wind parameters observed by many spacecraft over period 1963-86. From Hapgood et al. (Planet. Space Sci., 39, 410, 1991). Solar Wind Observed Near Earth Values of Solar-Wind Parameters Parameter Minimum Most Maximum Probable Velocity v (km/s) 250 370 2000× Number density n (cm-3) 683 Ram pressure rv2 (nPa)* 328 Magnetic field strength B 0 6 85 (nanoteslas) IMF Bz (nanoteslas) -31 0¤ 27 * 1 nPa = 1 nanoPascal = 10-9 Newtons/m2. Indicates at least one interval with B < 0.1 nT. ¤ Mean value was 0.014 nT, with a standard deviation of 3.3 nT.
    [Show full text]
  • THEMIS Observations of a Hot Flow Anomaly: Solar Wind, Magnetosheath, and Ground-Based Measurements J
    THEMIS observations of a hot flow anomaly: Solar wind, magnetosheath, and ground-based measurements J. Eastwood, D. Sibeck, V. Angelopoulos, T. Phan, S. Bale, J. Mcfadden, C. Cully, S. Mende, D. Larson, S. Frey, et al. To cite this version: J. Eastwood, D. Sibeck, V. Angelopoulos, T. Phan, S. Bale, et al.. THEMIS observations of a hot flow anomaly: Solar wind, magnetosheath, and ground-based measurements. Geophysical Research Letters, American Geophysical Union, 2008, 35 (17), pp.L17S03. 10.1029/2008GL033475. hal- 03086705 HAL Id: hal-03086705 https://hal.archives-ouvertes.fr/hal-03086705 Submitted on 23 Dec 2020 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L17S03, doi:10.1029/2008GL033475, 2008 THEMIS observations of a hot flow anomaly: Solar wind, magnetosheath, and ground-based measurements J. P. Eastwood,1 D. G. Sibeck,2 V. Angelopoulos,3 T. D. Phan,1 S. D. Bale,1,4 J. P. McFadden,1 C. M. Cully,5,6 S. B. Mende,1 D. Larson,1 S. Frey,1 C. W. Carlson,1 K.-H. Glassmeier,7 H. U. Auster,7 A. Roux,8 and O.
    [Show full text]
  • Chapter 3 – Astrospheres, Stellar Winds, and the Interstellar Medium
    3 Astrospheres, stellar winds, and the interstellar medium BRIAN E. WOOD AND JEFFREY L. LINSKY 3.1 The spatial extent of the solar wind While orbiting the Sun, the Earth and the other planers in our solar system are ex posed to an outflow of plasma from the Sun. This is the solar wi nd, which largely determines the particle environment of the planets, as well as their magnetospheric properties. Long-tenn exposure Lo the solar wind may have had signjficant cffectc; on planetary atmospheres in some cases. Mars is a particularly interesting example, as the Martian atmosphere was once much thicker than it is today, and the erosive effects of solar wind exposure are commonly believed to be responsible for itc; depletion (see Ch. 8; also Jakosky et al.. 1994; Atreya et al., 2013). The solar wind does not expand indefinitely. Eventually it runs into the inter­ stellar medium (ISM), the ex tre mely low-particle-density e nvironment that exists in between the stars. Our Sun is moving relative to the ISM that surrounds the planetary system, so we see a fl ow of interstellar matter in the heliocentric rest frame, coming from the direction of the constellation Ophiuchus. The interaction between the solar wind and the ISM flow determines the large-scale structure of our he liosphere. which basically defines the extent of the solar wind's reach into our Galactic environment. Other stars are naturally surrounded by their own "astro­ spheres" (allernatively "asterospheres"') defined by the strength of their own stellar winds, the nature of the ISM in their Galactic neighborhoods, and their rnlative motion.
    [Show full text]
  • Solar Wind Properties and Geospace Impact of Coronal Mass Ejection-Driven Sheath Regions: Variation and Driver Dependence E
    Solar Wind Properties and Geospace Impact of Coronal Mass Ejection-Driven Sheath Regions: Variation and Driver Dependence E. K. J. Kilpua, D. Fontaine, C. Moissard, M. Ala-lahti, E. Palmerio, E. Yordanova, S. Good, M. M. H. Kalliokoski, E. Lumme, A. Osmane, et al. To cite this version: E. K. J. Kilpua, D. Fontaine, C. Moissard, M. Ala-lahti, E. Palmerio, et al.. Solar Wind Properties and Geospace Impact of Coronal Mass Ejection-Driven Sheath Regions: Variation and Driver Dependence. Space Weather: The International Journal of Research and Applications, American Geophysical Union (AGU), 2019, 17 (8), pp.1257-1280. 10.1029/2019SW002217. hal-03087107 HAL Id: hal-03087107 https://hal.archives-ouvertes.fr/hal-03087107 Submitted on 23 Dec 2020 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. RESEARCH ARTICLE Solar Wind Properties and Geospace Impact of Coronal 10.1029/2019SW002217 Mass Ejection-Driven Sheath Regions: Variation and Key Points: Driver Dependence • Variation of interplanetary properties and geoeffectiveness of CME-driven sheaths and their dependence on the E. K. J. Kilpua1 , D. Fontaine2 , C. Moissard2 , M. Ala-Lahti1 , E. Palmerio1 , ejecta properties are determined E.
    [Show full text]
  • The CCNY Physics Department Newsletter Volume 12 Fall 2019 - Spring 2021
    CCNY PHYSICS The CCNY Physics Department Newsletter Volume 12 Fall 2019 - Spring 2021 Features CCNY Physics Club Wins Awards Quantum Computing and CCNY Physics Planetariums in a Pandemic 2 CCNY Physics Department Newsletter Welcome to the 2019-2021 CCNY Physics Department Newsletter. It has been quite a year, but our department has re- mained strong and devoted to both our students and our research programs. We’ve had a newsletter for a number of years, but this year we decided to spruce it up a bit and give all these hardworking faculty, staff, and students a nice record of the past year’s accom- plishements. Going foward, we will be releasing the newsletter in the fall of each year. So, enjoy this issue, and look forward to another in a few months. PHYSICS CLUB REMAINS VIBRANT, WINS AWARDS The CCNY Physics Club bustles with activity whether students are meeting face-to-face on campus, or associat- ing virtually abiding COVID-19 guidelines, and garners awards for their exemplary education and outreach activi- ties. First, the awards! Awards The 2020 Physics Club Execitives: Prof. Franco (Advisor), Michael Gaziani, Jireh Garcia, The Physics Club has recently garnered two major Lisa Chan, Matthew Kubikowski national awards. The Club, a chapter of the Society of is lauded for its vigorous STEM outreach program for un- Physics Students (SPS) received an Outstanding Chap- derprivileged elementary and middle school students. The ter Award from the SPS National Office for 2020. The Chapter runs this outreach program in partnership with designation, given to fewer than 15 percent of all SPS inner city schools and community groups, such as, Ham- chapters at colleges and universities in the United States ilton Grange Middle School, PS 325 Elementary School, and internationally, recognizes the CCNY chapter’s suc- America Scores, the New York Math Academy, the Bridge cessful “Physics Month” activities between October and Golf Foundation and the Eagle Academy of Harlem.
    [Show full text]
  • Atmospheric Escape from the TRAPPIST-1 Planets\Xmlpi{\\}
    Atmospheric escape from the TRAPPIST-1 planets and implications for habitability Chuanfei Donga,b,1, Meng Jinc, Manasvi Lingamd,e, Vladimir S. Airapetianf, Yingjuan Mag, and Bart van der Holsth aDepartment of Astrophysical Sciences, Princeton University, Princeton, NJ 08544; bPrinceton Center for Heliophysics, Princeton Plasma Physics Laboratory, Princeton University, Princeton, NJ 08544; cLockheed Martin Solar and Astrophysics Laboratory, Palo Alto, CA 94304; dInstitute for Theory and Computation, Harvard–Smithsonian Center for Astrophysics, Cambridge, MA 02138; eJohn A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138; fHeliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771; gInstitute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095; and hCenter for Space Environment Modeling, University of Michigan, Ann Arbor, MI 48109 Edited by Neta A. Bahcall, Princeton University, Princeton, NJ, and approved December 4, 2017 (received for review May 15, 2017) The presence of an atmosphere over sufficiently long timescales dence from our own Solar System suggests that the erosion of the is widely perceived as one of the most prominent criteria asso- atmosphere by the stellar wind plays a crucial role, especially for ciated with planetary surface habitability. We address the crucial Earth-sized planets where such losses constitute the dominant question of whether the seven Earth-sized planets transiting the mechanism (13, 14), and the same could also be true for exo- recently discovered ultracool dwarf star TRAPPIST-1 are capable of planets around M dwarfs (15, 16). Recent studies of atmospheric retaining their atmospheres. To this effect, we carry out numerical ion escape rates from Proxima b (and other M-dwarf exoplanets) simulations to characterize the stellar wind of TRAPPIST-1 and the also appear to indicate that the resulting ion losses are signifi- atmospheric ion escape rates for all of the seven planets.
    [Show full text]
  • 1 Stellar Winds and Magnetic Fields
    1 Stellar winds and magnetic fields by Viggo Hansteen The solar wind is responsible for maintaining the heliosphere, and for being the driving agent in the magnetospheres of the planets but also for being the agent by which the Sun has shed angular momentum during the eons since its formation. We can assume that other cool stars with active corona have similar winds that play the same roles for those stars (1). The decade since the launch of SOHO has seen a large change in our understanding of the themrally driven winds such as the solar wind, due to both theoretical, computational, and observational advances. Recent solar wind models are characterized by low coronal electron temperatures while proton, α-particle, and minor ion temperatures are expected to be quite high and perhaps anisotropic. This entails that the electric field is relatively unimportant and that one quite naturally is able to obtain solar wind outflow that has both a high asymptotic flow speed and maintaining a low mass flux. In this chapter we will explain why these changes have come about and outline the questions now facing thermal wind astrophysicists. The progress we have seen in the last decade is largely due observations made with instruments onboard Ulysses (2) and SOHO (3). These observations have spawned a new understanding of solar wind energetics, and the consideration of the chromosphere, corona, and solar wind as a unified system. We will begin by giving our own, highly biased, \pocket history" of solar wind theory highlighting the problems that had to be resolved in order to make the original Parker formulation of thermally driven winds conform with observational results.
    [Show full text]