studies of auroral x-ray backgrounds for high - KTH Particle and ...

studies of auroral x-ray backgrounds for high - KTH Particle and ...



Stefan Larsson (1) , Felix Ryde (2) , Nickolay Ivchenko (3) ,MarkPearce (2)

(1) Department of astronomy, Stockholm Unversity, AlbaNova, SE-106 91 Stockholm, Sweden


(2) Particle and Astroparticle Physics group, Department of Physics, KTH, AlbaNova, SE-106 91 Stockholm, Sweden

(3) Space and Plasma physics, School of Electrical Engineering, KTH, SE-10044 Stockholm, Sweden


Auroral X-ray emission was discovered in the 1950s

and has been studied with instruments on balloons,

rockets and satellites. While this radiation is of

prime interest for studies of space plasma in the

Earth’s magnetosphere the same radiation is also a

background for astrophysical observations made during

balloon flights at high latitudes. For such observations

it is necessary to monitor and understand

the properties of this radiation. This is particularly

true for hard X-ray polarimetry which is still an observationally

unexplored field. Instruments to measure

X-ray polarizations are being developed and will

probably first be flown on balloons. We discuss how

the auroral X-ray emission and in particular its polarization

properties may affect these observations

and whether these instruments also can provide information

about the high energy electrons producing

the X-ray aurora. Optical monitoring of the auroral

emissions to allow unambiguous relation of X-ray

background to aurora is also discussed.


Auroral X-rays are due to bremsstrahlung emitted

at 90 - 100 km height. This is well above the flight

altitude for balloon instruments (∼ 40 km). The

soft X-rays (below 10-20 keV) are efficiently absorbed

by photoelectric absorption in the upper atmosphere

and can therfore only be detected from space. The

harder X-rays are attenuated by Compton scattering

but penetrate deep enough to be observable at

balloon altitudes. The first detection of auroral X-

rays was made from balloons already in the 1950’s

[1] and satellite observations have been made with a

number of missions starting in the late 1960’s. The

most prominent emission is seen in soft X-rays, below

10 keV but some events are detected up 1 MeV.

The first measurement of MeV emission was made

with a balloon instrument launched from Esrange

in 1996 (”Kiruna-like events” [2]). With dedicated

long-term monitoring from space, in particular by

the Polar mission, there is now extensive data on auroral

high-energy emission. The various observations

made from balloons, rockets and satellites are still

complementary. Balloons provide long uninterupted

measurements around a single point which moves

very slowly compared to the more rapid variations of

the aurora. The data is therefore ideal for studies of

time variable phenomena. Rocket flights are shorter

but reach higher altitude. Also soft X-rays can therfore

be observed and a larger area can be viewed. In

addition, in-situ measurements of electrons can be

made along the rockets tradjectory. Satellites on the

other hand have the advantage that they can view simultaneously

the full, or a large part, of the polar cap

region. Compared to optical auroral observations, X-

ray observations are more difficult to perform but can

be made also during day time and most importantly

provide information about the higher energy electron

component (> 10 keV) for which little information

can be obtained from the optical emission, with their

intensities dominated by the electrons with energies

where the bulk of the energy flux occurs.

In this paper we will discuss the known observational

properties of auroral X-rays, how they are produced

and what their polarization properties are predicted

to be. We will then look at the relevance of this

radiation component as a background source for astrophysical

X-ray instruments on balloon platforms.


Observer, PoGOLite [3]. It is planned to launch

PoGOLite on one or more long duration flights from

Esrange to northern Canada. This is a flight trajectory

which takes the instrument through the auroral




2.1 Electron bombardment

Most aurora occur at latitudes between the geomagnetic

closed-dipole field and the region with open

field lines, approximately 65 − 70 ◦ . The aurora is

caused by an increased flux of extra-terrestial electrons

with energies of a few keV bombarding the

atmosphere [4]. These have been accelerated at

an altitude of 2000-10000 km in a static potential

drop streaming down parallel to the magnetic field

lines. The energy spectrum of the accelerated, precipitating

electrons is well described by an accelerated

Maxwellian distribution (passed through a fixed

field aligned voltage drop) around 1-10 keV and a

power law distribution at lower energies originating

from the pre-accelerated distribution. The acceleration

potential is related to the auroral current density

and electron temperature, a dependence that is

described by the Knight relation [5]. Furthermore,

the largest acceleration potentials are observed on

the night-side. The electrons penetrate the atmosphere

losing energy by ionizing and exciting atoms

and molecules (giving rise to visible aurora) and by

emitting bremsstrahlung in the X-rays. It was indeed

X-ray observations (10-100 keV) that first indicated

that energetic extraterrestial electrons contribute to

the aurora since the observations only could be explained

by electron emitting bremsstrahlung in the

auroral region at 100 km height. Depending on the

size of the pitch angle (angle between the magnetic

field and the velocity) the gyration of the electron

will cause it to emit cyclotron emission at lower frequencies

with varying strength.

At somewhat lower latitudes (< 65 ◦ ), where the electrons

are trapped in the dipole magnetic field, diffuse

aurora is also created by pitch-angle scattering

of the electrons into the pitch-angle loss-cone (which

includes the pitch angles for which the electrons no

longer are reflected and thereby trapped). The fluxes

of the trapped electrons are enhanced during the geomagnetic

storms. The details of the acceleration

and pitch-angle scattering mechanisms, and their interplay

are still subject of active research.

For X-ray emission, the essential part of the electron

spectrum is the Maxwellian peak distribution. The

bremsstrahlung spectrum in the 20-200 keV range is

thus described by an exponential cut-off (∝ e E/E0 ),

which can be fitted to yield an e-folding energy, E 0 ,

giving the temperature of the electron distribution.

2.2 Bremsstrahlung

As a charged particle passes a nucleus it is accelerated

in the Coulomb field and will therefore emit

bremsstrahlung. The total spectrum will depend on

the cross-section’s dependence on energy and on the

loss rates as the electrons traverse parts of the atmosphere.

Beside radiation losses it will also lose energy

through ionizing collisions. For electrons with

energy ɛ 1 ∼ 100 keV approximately 10 −3 of the

energy is lost due to bremstrahlung. An electron

with energy ɛ 1 will emit an energy spectrum that

will be flat with a maximal energy hν 1 = ɛ 1 . The

bremstrahlung spectrum therefore yields the instantaneous

spectrum of the electron energies 1 . To arrive

at the initial, incoming spectrum outside the atmosphere

(incident energy ɛ 0 ) one has to integrate the

relative radiation losses over the traversed trajectory.

It can be shown [7] that the total amount of energy

emitted in bremstrahlung by an electron entering the

atmosphere with energy ɛ 0 is given by

ɛ rad = (ɛ 0/MeV) 2



Together with the observed energy-fluence of the

photons (the exponential cut-off spectrum yielding

an upper limit for ɛ 0 ) then gives a rough estimate of

the incident electron flux.

2.3 Polarization of X-rays

Early observations have indicated strongly

anisotropic pitch angle distributions of the incident

electrons in that there is a sharp peak

around zero degrees [8]. This, together with the

above discussion, motivates the simplified picture

of monoenergetic electrons travelling along the

geomagnetic field lines before reaching the lower

atmosphere. Such an electron beam produces highly

polarized bremsstrahlung emission. Reference [9]

studied such a scenario theoretically and calculated

the degree of polarization, Π, as a function of

viewing angle relative to the electron stream and

pitch angle (angle between the magnetic field and

the velocity of the electron). Π was shown to

strongly depend on the pitch angle. The largest

calculated polarization, Π ∼ 80% was found for

small pitch angles and a viewing angle of ∼ 90 ◦ .As

the pitch angle increases, Π decreases significantly;

at 60 ◦ pitch angle, Π max ∼ 0.3.

The pitch-angles of the precipitating electrons are a

complicated subject, and the above description is by

necessity a strongly simplified picture, yet illustrative.

As the electrons propagate into the region of

stronger electric fields, their pitch-angle is increased

due to the magnetic momentum conservation. A distribution

of the pitch angles will drastically decrease

1 Bremsstrahlung from the power-law distribution of electrons

N(E) ∝ E −x yields a photon spectrum, of the same

power-law form N γ(hν) ∝ hν −x (in photon flux [6]

the degree of polarization. The bremsstrahlung flux

is also expected to increase at lower altitudes [9]. The

observed polarization will of course also decrease due

to the geometry and morphology of the magnetic

field and electron stream; a purely vertical stream

will give zero polarization in the vertical direction

and maximal polarization perpendicular to it.



3.1 Occurrence rate

A number of long duration balloon flights (1-2 weeks)

have made measurements of auroral emission in the

hard X-ray range [10,11,12]. From these we can make

a rough estimate of how large fraction of the observing

time that an instrument like PoGOLite might be

affected by auroral emission. We use the results from

a two week flight with MAXIS in Antarctica in January

2000 for this. The measured X-ray fluxes in two

energy bands (20-1300 keV and 180-1300 keV) have

been published [12]. No X-ray activity was detected

during the initial period when the balloon’s latitude

was > 71 ◦ . Later, at lower latitudes and low geomagnetic

activity index [K p ∼ 0 − 3] only sporadic

activity was detected. During a moderately strong

geomagnetic storm (D st,max = −91 nT) a more continuous

X-ray bursting was detected over a 4 day

period. The Kiruna-like MeV events have a typical

duration of tens of minutes.

Based on these and the other available observations

we expect that PoGOLite will be exposed to

strong auroral X-rays for 5 - 10% of the time during

a long duration flight from Esrange to northern

Canada. The observations also suggest [10 and references

therein] that the high-energy radiation is seen

mostly near the trapping region and rarely on the

open field lines at higher latitudes.

3.2 Intensity

The flux of the auroral emission is strongly variable.

The available balloon measurements suggests that

a bright X-ray aurora has a flux of ∼ 0.1 photons

s −1 cm −2 sr −1 keV −1 at 40 keV. An X-ray instrument

pointing to an astrophysical source will then

also measure the sky background over some area of

the sky. PoGOLite is a collimated instrument with

a field-of-view of about 5 square degrees so in this

case the auroral background flux is equal to the flux

of an approximately 30 mCrab source. This should

be regarded as a typical bright event. The maximum

fluxes detected by the long duration flights referred

to above were at least 3 or 4 times this level. We can

therefore expect that PoGOLite will be exposed to

auroral X-rays with fluxes of a few tens of mCrabs

during something like 5 - 10 % of the flight and during

shorter times the fluxes may reach or even exceed

100 mCrabs. The exposure is also expected to

decrease during the last part of the flight which is

well inside the auroral oval.

4. Satellite observations

The auroral emission seen by a satellite from space

is not the same as that seen from a balloon below

the emission region. Available satellite observations

are nontheless useful for understanding the distribution

and occurance rate of X-ray aurora. The Polar

Ionospheric X-ray Imaging Experiment (PIXIE) on

board the NASA/GGS POLAR spacecraft provided

imaging of the auroral region in the 2 - 60 keV band

from 1996 to 2002. From such data the average and

median flux in the 2-12 keV range in one-hour bins

has been analysed [13]. The median flux was found

to be noticeably lower than the average flux, which

indicates that it is periods of strong auroral activity

rather than continued activity that is the dominating

pattern. These are the occasional geomagnetic

storms, when the population of trapped electrons is

strongly enhanced. From these results one can also

see that the flux has a maximum at local midnight.

The results also show a strong correlation between

the X-ray flux and the Auroral kilometric radiation

(AKR), which indicates that they have a similar origin.

The source of the AKR is a plasma instability

of specific shapes in electron distribution function.

The shapes (with sharp gradients in perpendicular

velocities) are a kind of loss-cone distributions.

It has been shown [13] that the average X-ray flux

rises exponentially with the K p index, which measures

the geomagnetic activity. Furthermore the average

magnetic latitude is a function of geomagnetic

activity. During quiet periods the magnetic latitude

of the X-ray aurora is ∼ 68 ◦ ,whichmovestolower

values at higher activities.


5.1 Auroral X-ray background

We expect strong auroral X-ray emission for approximately

5-10% of the flight time during an Esrange

flight. This radiation is mainly seen near the trapping

region and rarely at higher latitudes. The fluxes

are expected to be of a few tens of mCrabs. However,

during shorter spikes the fluxes may reach or

even exceed 100 mCrabs. This should be compared

to the fluxes of the astrophysical targets planned for

PoGOLite. These have fluxes in the range ∼ 100-

1000 mCrabs.

An additional factor to take into account is the solar

activity cycle. The largest auroral activity occurs

near solar maximum, the next of which is expected

to be around 2011. Although this increases the X-ray

aurora it also decreases the cosmic ray background.

5.2 Possibilities of Auroral X-ray detections

PoGOLite detection of the aurora is to be regarded

as background events for the measurements of celestial

sources planned. However, these measurements

are indeed of interest in themselves for aurora science

and indeed are unique as polarization measurements.

Measurements of the variability and the photon

spectrum of auroral X-rays with PoGOLite will

be able to reveal the nature of the precipitating electron

flux, their energy and angular distributions, as

well as the acceleration mechanisms involved. Such

measurements will be very helpful in distinguishing

between temporal and spatial effects which otherwise

is difficult. The slow motion of the balloon makes it

essentially a stationary observatory, whereas satellites

traverse the region of interest very fast. In particular,

polarization measurements of this emission

will provide unique results. The strength will directly

reveal the pitch angle distribution of the incident

electrons, thereby giving essential clues to the

acceleration processes at play, which in many cases

is very difficult to measure.

5.3 Suggestion for aurora monitoring on


The PoGOLite mission would indeed benefit from

having auroral diagnostics onboard, even if the aurora

is not targeted specifically. A strong X-ray aurora

will be seen as an increase of detected events

in the instrument and in the anticoincidence system.

To directly indicate whether there is aurora in the

field of view, and what are the energy characteristics

of the precipitation a couple of photometers

with narrow passband filters may be included in the

instrumentation. This would also be of interest for

observations of pulsating aurora, which is the most

relevant target for balloon observations.

Information about geomagnetic storm activity is provided

by, among others, the D st index, which is a

global index derived from magnetometers placed all

around the globe at the equator. In addition, there

are many magnetic observatories in the auroral region,

but mainly on land. Since the distribution is

rather scarce a fluxgate magnetometer may also be

included in the aurora diagnostic package onboard



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