761658S Ionospheric Physics
Lecture 1
Anita Aikio
University of Oulu, Finland
Ionosphere
• 80 - 1000 km altitude
• Partly ionized upper
atmosphere
• Strongly affected by the Sun
Sun
• Themeandistancefrom theSun to
theEarthiscalled1astronomical
unitAU=150milj.km.
• Thesolardiameterofthevisible disc
is1.4·106 km,whichisabout109
timesthediameteroftheEarthand
theviewinganglefromtheEarthis
about0.5degree.
• Ittakesabout8minfrom thelight to
reachtheEarth.
• Themedium massdensityisabout
1.4timesthedensity ofwater.
• TheSun irradiatestheEarthwith
1367± 3W/m2 (thesolarconstant),
abouthalfofwhichreachesthe
surfaceoftheEarth.
solarflare
Solar rotation
• Sunhasdifferentialrotation:rotationisfastestattheequator:26.9
days,andslowerathigherlatitudes,atthepoleitis32.4days.
• Thesynodic(observedfromtheEarth)rotationperiodindays:
Tsy=26.9+5.4sin2λ ,
where λ isthesolarlatitude(λ iszeroattheequator)
• Thesolarrotationsareassignedwithnumbers,Bartelsnumbers,
accordingtoarotationperiodof27dayscorrespondingtoalatitude
of8o.InthissystemtherotationstartingonFebruary8,1832is
number1.
• Bartelsrotationnumberssince1832:Sep2013thenumberis2458.
Structure of the Sun
• Core:T~15·106 K=>fusion reaction
411H->42He+2e++2νe+2γ+24.64MeV
• Fusion=> Thethermalenergyand
pressurepreventstheSuntocollapse
becauseofitsowngravity.
• IntheRadiativezone 0.25–0.8Rs the
energytransferoutwardsiscarriedby
γ and Xrayphotons. Becauseofthe
numerous absorption/emission processes
ofthephotons, ittakes105–107 yearsfor
theradiationtoreachthesolarsurface.
• IntheConvectivezone,thetemperature
hasdecreasedsomuchthatatomsstart
toform fromnucleiandelectrons.The
warmplasmainthebottomof thezone
raisesup,cools, anddescends down=>
supercells.Thesmallerstructuresof
supercellsatthesolarsurfacearecalled
granules (<1000km).
• ThePhotosphere isthebright surfaceof
theSun.
• Thesolaratmosphere consistsofthe
cromosphere andthecorona.
Photosphere
•
•
•
•
•
•
•
The visible surface of the Sun
Small thickness, dense and nontransparent layer
Thickness only ~ 500 km
Density ~ 1017 cm-3
Temperature: Teff ~ 5780 K
– bottom, T ~ 7000 K
– top, T ~ 4500 K
Sunspots are visible as dark
spots.
Irradiates visble light with a
continuous spectrum (Planck’s
radiation law) and contains
absorption lines
Cromosphere
•
•
•
•
•
The lowest part of the solar
atmosphere, where temperature
starts to increase
Low density ~ 1011 cm-3
Transparent layer
Thickness ~ 2000 km
Temperature increase
– slowly the first 1500 km to
9000 K
– fast in the transition zone
about 500 km thick to ~ 106 K
Lusaka, Zambia, June 21, 2001
• During the solar eclipse, the spectrum of the cromosphere can be
measured. It contains several strong emission lines, e.g. hydrogen
Balmer series Hα (656.3 nm) and hydrogen Lyman lines (Lyα 121.6 nm ja
Lyβ 102.5 nm) in the EUV regime.
Corona
• Hot plasma, T ~ 2· 106 K,
ionization degree 100%
• The outward expanding
plasma forms the solar
wind.
• High ionization degree of
the different atoms (Fe, Si,
Mg, O,...) and their
emission lines.
• Because te temperature
increases with height, the
emission lines originate
from different altitudes.
8.1. STRUCTURE OF THECorona
SOLAR ATMOSPHERE
201
Corona during sunspot minimum Corona during sunspot maximum
Figure (clear
8.5: Solar
at (left) solar minimum and (right) solar maximum.
helmetcorona
streamer)
Observations:
of white
The black sphere at the center is the
occulting The
discroots
of the
coronagraph.
light
streamers are associated with
Vertical axis points to the heliographic
north.
emissions of X rays from the
photosphere.
contributions by the toroidal field (see Figure 8.5). There are also coronal
holes during maximum years but they are distributed rather around the
equator (not only around poles) and they do follow the systematic polarity
rule that is typical for polar fields during solar minima. The lifetime of
Temperature in solar atmosphere
Cromosphere/transition region
Underconditions (temperatures
above50,000K)thatexistinthe
Sun'satmosphere, helium atoms
areionizedtoformHeIIions.These
ionsemitextremeultraviolet(EUV)
radiationatawavelengthof30.4
nm(304Å).Radiationatthis
wavelengthallowsscientiststosee
structuresandprocessesinthe
transitionregion, theboundary
betweentheSun's lower
atmosphere(chromosphere) andits
upper atmosphere(corona). These
high energyUVemissions are
blocked byEarth'satmosphere, so
orbiting solartelescopeson
satellitesabovetheatmosphere
mustbeused toviewtheSunat
thiswavelength.
I
T of solar atmosphere and EUV emissions
EUV emissions
•
•
•
TheUVradiationregionisusuallydividedintotwoparts:thefar
ultraviolet(FUV)regionbetween100and200nmandtheextreme
ultravioletregion(EUV)between10and100nm.
TheEUVradiationoriginatesfromthechromosphere.IntheEUV
regimethereare anumberofstrongemissionlinessuchasthe
hydrogenLymanlinesat121.6nm(Lyα),102.5nm(Lyβ),theHeIline
at58.4nmandHeIIlineat30.4nm.Also,emissionsfromneutral
ionsuptoveryhighionizationlevels,e.g.FeXIIat19.5nminthe
solarcoronaaremeasurede.g.bytheSOHOspacecraft.
Whilethesolarspectrumbetween200and3200nm(visibleandIR)
isratherstableduringasolarcycle,theUVlinesandtheX-ray
emissionsaremorevariableandoftenenhancedduringthesolar
flareevents.
EUV and X rays
Solarflarestakeplaceinthesolar
coronaandchromosphere, heating
plasmatotensofmillions of kelvins
andacceleratingtheresulting
electrons,protons andheavierions to
nearthespeedoflight.Theyproduce
electromagneticradiationacrossthe
electromagneticspectrumatall
wavelengthsfrom long-waveradioto
XraysandGammarays.Mostflares
occuraroundsunspots, where
intensemagneticfieldsemergefrom
theSun'ssurfaceintothecorona.The
energyefficiencyassociatedwith
solarflaresmaytakeseveralhoursor
evendaystobuild up,but mostflares
takeonlyamatterofminutes to
releasetheirenergy
Visible spectrum of the Sun
57 8 0
O2
• Solarspectrumcorresponds toablackbody atatemperatureof5780K(coming
from the photosphere)
• Redpartofthefigure showsthespectrumatsealevel
Total solar irradiance
1995
2000
2005
2010
2015
1364
VIRGO
ACRIM II
HF
1990
ACRIM I
HF
HF
ACRIM I
1368
1985
1362
1366
1360
Min21/22
1364
Min22/23
Min23/24
1358
Average TSI, org & new: 1366.05 & 1361.11 Wm−2 −2
Minimum 21/22, org & new: 1365.68 & 1360.73 Wm−2
Minimum 23/24, org & new: 1365.44 & 1360.50 Wm
1362
1980
1985
1990
1995
2000
2005
2010
TSI (Wm−2, new VIRGO scale)
TSI (Wm−2, original VIRGO scale)
1980
2015
• Thetotalsolarirradiance(TSI)istheamountofsolarradiativeenergyincidenton the
Earth'supper atmosphere.TSIvariationswereundetectableuntilsatelliteobservations
beganinlate1978.Averagevalueofabout1361.5W/m2 hasbeenassigned tothe"solar
constant",butTSIvariesinphasewiththesolarmagneticactivitycyclewithan
amplitude ofabout0.1%.TSIishigher atsolarmaximum,eventhough sunspots are
darker(cooler) thantheaveragephotosphere. Thatiscausedbymagnetized structures
otherthansunspots during solarmaxima,suchasbright faculae.
.3. SOLAR SPECTRUM
Solar radio emissions
17
• Superposed onthethermal
background solarradioemissions,
emissions associatedwith solar
flaresandother phenomena take
place.During strongsolar
disturbances theradioemissions
canexceedthequiet-timelevelby
severalordersofmagnitude.
• Slowlychanging component, the
S-component (maxnear10cm),
followsthe11-yearsunspot cycle.
Itisalsoratherwellcorrelated
withthefluxesoftheEUVandXrayradiation.
• The10.7cm(2.8GHz)radiowave
fluxismeasuredatDominion
AstronomicalObservatory, Ottawa
andrepresentasF10.7inunitsof
10-22 W/(m2 Hz)reduced to1AU.
• http://www.heliotown.com/Radio
_Sun_Introduction.html
igure 1.2: Solar radio emissions. Dots and the solid curve represent the
Sunspot number and 11-yr solar cycle
Solar radio flux index F10.7
Coronal mass
ejection (and
the NEAT
comet) as
measured by
the LASCOinstrument of
the SOHO
satellite
Solar origin of aurora in the ionosphere
Aurora photographed from the International Space Station