Energetic (>10keV) and relativistic electron (>500keV ... · Energetic (>10keV) and relativistic electron (>500keV) precipitation into the mesosphere - evidence and limitations Craig

Energetic (>10keV) and relativistic electron (>500keV) precipitation into the mesosphere

- evidence and limitations

Craig J. Rodger

Department of Physics

University of Otago

Dunedin

NEW ZEALAND

HEPPA/SOLARIS 2012

Energetic particle measurements 10:45-11:15 Wednesday 10 October 2012

Craig J. Rodger1, and M. A. Clilverd2

1. Physics Department, University of Otago, Dunedin, New Zealand.

2. British Antarctic Survey (NERC), Cambridge, United Kingdom.

Basic structure of the Van Allen belts

In 1958 the first US

satellites were launched

into orbit carrying Geiger

counters. Explorer I and

Explorer III discovered

the Van Allen radiation

belts.

On average the belts are

structured with an inner

and outer belt, separated

by the “slot”.

Adapted from Rodger and Clilverd, Nature, vol. 452, 2008. Explorer 1 – post

launch press briefing.

Particle access to the upper atmosphere

Radiation Belt

Precipitation

Losses: The outer radiation

belt deposits energy into the

polar atmosphere in both the

Antarctic and Antarctic (primarily

due to wave-particle interactions with

ULF & VLF waves), and thus is of

most interest to this community.

The potential importance of particle precipitation

Particle precipitation is one of the routes by which the Sun can link to the

climate – energetic electrons and protons can change the atmospheric

chemistry. And in an environment where humanity is changing the

climate, and the polar ozone levels, we need to know about the “natural”

variation too!

Particle precipitation

Production of NOx and HOx

Change in dynamics

mesosphere & stratosphere

Destruction of mesospheric

and upper stratospheric O3

“Climate”

Plus of course the

interest in

precipitation from a

strictly radiation belt

physics viewpoint.

(Probably)

Atmospheric Chemistry Experiment Fourier Transform Spectrometer

(ACE-FTS) solar occultation observations of descending NOx in the Arctic

winters. The primary source for the indirect effect is believed to be low-

energy particle precipitation (i.e., auroral electrons (<10keV) and protons

(<500keV) producing NOx at ~100km or so.

Randall et al., Geophys. Res.

Lett., 36, doi:10.1029/

2009GL039706, 2009.

Evidence that EEP might “matter”? – “Indirect”

NO observations from the British Antarctic Survey radiometer located

at Troll station, Antarctica (65° geomagnetic latitude). NO increases at

~75km by 2-3 orders of magnitude due to multiple days of ~300keV

precipitation. This effect is far too fast to explain through transport.

Newnham et al., Geophys.

Res. Lett.,

doi:10.1029/2011GL049199,

2011.

Evidence that EEP might “matter”? – “Direct” (NOx)

Correlation between POES satellite precipitating electron count rates and OH

(part of the HOx family). In one third of the months from 2004-2009 the

observed OH variation is best explained by EEP, with direct OH-increases

measured to as low as 52 km altitude (~3MeV electrons!).

Andersson et al. (2012),

J. Geophys. Res.,

10.1029/2011JD01724.

Evidence that EEP might “matter”? – “Direct” (HOx)

+

Evidence that EEP might “matter”? – “Direct” (O3)

AARDDVARK subionospheric VLF observations DEMETER DLC

Precipitation fluxes required to reproduce the changes in AARDDVARK-observed subionospheric propagation (NAA -> CAM).

Peak Fluxes:

8000 el. cm-2s-1 at midday

800 cm-2s-1 at midnight.

Rodger et al. (2010), J.

Geophys. Res.,

10.1029/2010JA015599.

Model the neutral atmosphere changes: NOx

Energetic electron precipitation results in the enhancement of odd nitrogen (NOx) and odd hydrogen (HOx), which play a key role in the ozone balance of the middle atmosphere. Using SIC, we can look at the electron precipitation produced changes, during this storm period.

Factor of 5-6 increase that is most significant in the ~65-85 km altitude range

The NOx increase builds up primarily across the time-span when the >150 keV electron precipitation fluxes peak, and then start to recover due to photodisocciation.

Looks impressive, but it is

important?

Day of September 2005 Day of September 2005

Model the neutral atmosphere changes: O3

NOx and HOx increases caused by energetic particle precipitation have been associated with in-situ ozone loss in the polar middle atmosphere. This has been experimentally observed during Solar Proton Events. So what about for electron precipitation?

In the case studied here there is an essentially insignificant level of ozone loss (<1% most of the time, brief peaks at ~3%).

Model the neutral atmosphere changes: NOx

However, we considered the Northern Hemisphere during late summer-early autumn. The dark atmosphere, particularly the polar winter atmosphere, is very different. So lets take a Southern Hemisphere case (same L-shell = magnetic latitude) in deep SH winter.

While the percentage change is not so big, the absolute changes are larger, and persist longer.

Again, looks a bit impressive, but it

is important?

Model the neutral atmosphere changes: O3

We know the response to particle precipitation is dependent upon hemisphere and season (this has also been experimentally observed during Solar Proton Events). So if we look at the Southern Hemisphere and winter, then yes, it’s a very different picture!

In this case, because of seasonal asymmetries in background chemical composition, we get a significant in-situ O3 change! So experimental and modelling evidence that “direct” EEP is likely significant.

This is similar magnitude to that modelled and observed during solar proton events!

Understanding the Radiation Belts - Particle Motion A charged particle trapped in the Radiation Belt experiences three basic motions – cyclical (around the field line), bounce motion (between the

hemispheres) and drift around the Earth.

So we need

to

understand

electron

precipitation

much better!

Understanding the Radiation Belts -The Importance of the Pitch Angle

The long term fate is determined by the pitch angle (α) of a radiation belt particle at the geomagnetic equator. For example

α = 90° is trapped at the geomag. equator.

α = 0° will strike the Earth’s surface (and thus lost)

In reality, the majority of Radiation Belt electrons have pitch angles between this range (i.e. neither α = 0° nor α = 90°), and so bounce from hemisphere to hemisphere passing through the geomagnetic equator.


While an equatorial pitch angles of α = 0° will strike the Earth, an electron which mirrored at 1m above the ground would also be lost by colliding with the atmosphere. In practise there is a range of pitch angles which will be quickly lost through atmospheric collisions. The threshold is taken as ~100km altitude, and this range of pitch angles defines the loss cone with the outer edge the “loss cone angle”, αLC.

Any electron which starts with a pitch angle smaller than αLC (or is scattered into that range) will be rapidly loss – within a few bounces at most.


BUT the mirror height (and hence the width of the loss cone) depends on the strength of the magnetic field, and this is not constant at the surface of the Earth or at satellite altitudes.

Thus the width of loss cone angle, αLC, varies with latitude, and longitude (and altitude, but we normally reference α to the geomagnetic equator).

SAMA (or

SAA)


So in practise there are two loss cones at any given location – the “local” bounce loss cone αLC, and the “drift” loss cone αDLC, which is the minimum value of αLC for a given magnetic drift shell (and thus L) and tends to be located in the South Atlantic-ish region.

Note that most LEO spacecraft are normally measuring pitch angles near the αDLC. So these electrons are either certain to be lost soon or are not far off being scattered into the drift loss cone or bounce loss cone.

Example of pitch angle scattering into the Drift Loss Cone

NWC

Dunedin

We will focus on one of the

world’s most powerful VLF

transmitters. The US Navy

transmitter NWC operates at

19.8kHz and radiates 1 MW.

The Otago Space Physics group makes

continuous measurements of the amplitude

and phase of NWC-transmissions from our

antenna in Dunedin.

The DLC is strongly affected by a Tx in my “backyard”

Altitude

= 6200 km

Altitude

= 1800 km

Altitude

= 11 km

733 m

It is VERY large.

The 6 outer

towers are 364m

high, the inner 6

are 304 m, and

the central tower

is 387m high.

Altitude

= 3 km

Here we take the ratio of 5 months of >100keV observations from the

4 NOAA POES spacecraft (N-15,-16,-17, & -18) when NWC was on

and 5 months when it was off. NOT such a subtle feature anymore!

Example of pitch angle scattering into the Drift Loss Cone

Satellite Observations of EEP

Space Based Observations of

EEP from DEMETER

Orbit: ~710km Sun synchronous.

IDP measures mostly DLC

electrons from ~70 keV to

~2.2 MeV in 128 steps.


Space Based Observations of

REP from SAMPEX

Orbit: 520 × 670 km altitude

HILT measures mostly DLC

electrons with energies >1 MeV.

While these electrons are trapped, or are going to be lost in the SAMA

(and not locally), these fluxes are much much larger than those in the

BLC.

Not useful for driving a climate model, but useful for working out context

and looking for changes – confirming that “fresh” particles are available

for loss!

Why do we care about just trapped and DLC electron fluxes?

If one looks at the “just trapped” >100keV electrons measured from Low

Earth orbit you can see huge variability (many orders of magnitude).

DLC e- fluxes

1998-2011


Space Based Observations of EEP

from POES

Orbit: ~835 km Sun synchronous.

While suffering from numerous

limitations, POES is the most widely

used source of space based EEP

observations (and includes BLC) with

really long datasets available!

And MetOp-B

SEM-2 data to be

online from early

October!

Radiation Belt Issues – POES measuring Loss Cones

POES “maybe just trapped” (90º) telescope

POES “bounce loss cone” (0º) telescope

Losses: the POES database

All of the Space Environment Monitor (SEM-2) data is available for download

from the NOAA website, in ~2s resolution CDF format.

Contamination information from

Yando et al. (2011), J. Geophys.

Res., 116, A10231,

doi:10.1029/2011JA016671.

So we can remove the solar proton events seen with the P5 telescopes and the

P7omni channel, and hence use the P6 and P6omni as REP monitors.

The proton channels (P1-P4) are really providing data for auroral altitudes

(>100km), and then for SPE events.

The electron channels are well focused for “direct” EEP impacts at altitudes

below 100km.

The problems with POES SEM-2 e- data

The POES observations look perfect for our science needs. There is a lack of electron

precipitation data for energies >10 kev (that is altitudes below ~100 km), and the

POES SEM-2 does actually observe inside the Bounce Loss Cone. BUT we need to

beware as there are issues, caveats, and unknows to trip us up when we make use of

the electron observations.

Certainly, POES can be used to provide context and evidence that energetic electron

precipitation is happening (and probably relativistic electron precipitation too, if it is

intense enough as the response in P6 is pretty weak).

At least this point it is not really clear we can use the POES electron precipitation

data to feed global chemistry climate models to establish the significance of EEP.

There is a danger of “garbage in” = “garbage out” making the conclusions spurious.

Some basic issues.

1. The energy converge is poor (only >30, >100 & >300keV integral fluxes).

2. We can’t be certain what magnitude flux is actually going into the atmosphere.

3. The sensitivity of the instrument is pretty poor, so that “medium” levels of

>100keV and >300keV precipitation in quiet(ish) periods are normally lost/missed.

We are working on these issues, but its not as easy to fix or work-around as it

sounds!

Electron Instruments Lie when High Energy Protons Are Present

The last time we were in Boulder (HEPPA, 2009) Janet Green from

NOAA warned the POES users about the dangers of using the POES

SEM-2 electron measurements during solar proton events.

Some evidence – lets first look at a “quiet” situation (no SPE, no

strong EEP) as NOAA-15 crosses the north and south poles.


The last time we were in Boulder (HEPPA 2009) Janet Green from



Some evidence – finally lets look at a “EEP” situation (no SPE,

active EEP) as NOAA-15 crosses the north and south poles.


The last time we were in Boulder (HEPPA 2009) Janet Green from



Some evidence – finally lets look at a “SPE” situation (strong Solar

Proton Events) as N-15 crosses the north and south poles. BAD!

SPE cause

overestimation of

>30keV electron

fluxes by 103 or

more. Unusable.


We are used to seeing BIG fluxes in the SAMA, suggesting this is the

most important feature for EEP. However, this is totally unclear as it’s a

proton feature – the inner radiation belt high energy protons make it

look like that feature is really significant.

Plot below is a 10 year average for very low geomagnetic conditions with and without p+

0° telescope

(BLC=

“being lost”)

Low energy p+ contamination of e1,e2,e3

Janet Green (and the POES instrument databook) also warns us that the

electron channels can be contaminated by low energy “auroral” protons.

Earlier I tested this to answer this:

What percentage of the POES/SEM-2 medium electron observations

are untrustworthy? How does this vary with geomagnetic activity?

Trapped are mostly OK, while precipitating is affected, particularly

when disturbed. This has some implications for the existing

climatologies. But it can be corrected for and Janet has developed an

algorithm which can be downloaded from VirBO. NOTE the

correction impact is actually smallish (factor of 2-5, in reality).

Rodger et al., J.

Geophys. Res., 115,

A04202, doi:10.1029/

2008JA014023.

POES 0° telescopes observe the Loss Cones, Right?

POES “bounce loss cone” (0º) telescope

We would like it if these telescopes

measured ALL the flux in the BLC at

that location, and NOTHING ELSE.

The SEM-2 0° electron telescopes

observe part of the BLC. In reality,

after we correct for low E protons (and

ignore SPE periods) we need to find a way to

deal with the “geometry factor for the

0° electron telescopes.

POES: geometry correction factor - Example

Geometric factor at this

location = 13

(i.e., we view only one

thirteenth of the BLC)

BLC DLC

0° telescope viewing region size

POES 0° telescope

observations for a field line

which joins the atmosphere

at 100km altitude above

Halley station Antarctica

(L=4.5)

POES: geometry correction factor for the fraction of the BLC observed

1

So a straight geometry factor correction suggests the 0° telescopes underestimate the electron

fluxes by 10-20 times (and here we have ignored the distribution of flux inside the loss cone,

which will likely make this situation worse as Richard Horne has explained in his talk).

More information on this “geometry factor” in the poster by Mark Clilverd

Wednesday

afternoon poster

session (A2).

Here we look at

different events and

try and see if we can

find evidence of this

“geometry factor”

and related

corrections!

How to turn 3 integral electron fluxes into a reliable EEP flux?

From Yando et al. (2011), J.

Geophys. Res., 116, A10231,

doi:10.1029/2011JA016671.

From Clilverd et al.

(2012), J. Geophys.

Res., (in review),

doi:10.1029/2012JA01

8175.

So what is POES data good for? CONTEXT!

POES data is very good for looking for changes

in conditions, and this is valid for most situations

(although not inside Solar Proton Events). So for

example:

When the Galaxy-15 communications satellite transponder failed at 0948UT

on 5 April 2010 and the satellite was turned into a “zombie”, we could use

POES data to quickly spot the culprit. A very strong injection of “hot”

electrons is seen starting at 0902UT and lasting till ~10UT in a substorm.

Galaxy-15 magnetic latitude

From Clilverd et al. (2012), J.

Geophys. Res., (in review),

doi:10.1029/2012JA018175.

Some extra facts! (no time to expand)

Energetic particle precipitation is not linearly linked to

geomagnetic indices, i.e. you can get lots of EEP with relatively

small geomagnetic disturbances.

Generally large geomagnetic disturbances will be linked to lots of

EEP and small disturbance levels will have smallish (but non

zero) precipitation magnitudes. BUTin addition we do see events

which are not strictly “geomagnetic storms” after which there are

really big precipitation levels.

From Hendry et al., AGU

Monograph "Dynamics of the

Earth's Radiation Belts and Inner

Magnetosphere", (in press),

doi:10.1029/2012BK001299, 2012.

We have been awarded a New Zealand

Marsden Funded project to investigate this

very issue:

“Evaluating the Impact of Excess Ionization

on the Atmosphere (EI EI A)”

My main support comes from:

Significant support, including for

my PostDoc Ian Whittaker who is

working on POES spectral fitting,

has been provided by the EU FP7

PLASMON project.

And I better not forget

Conclusions • Energetic electron precipitation (EEP) appears to be a significant part of the

variability of the mesosphere (i.e. “directly” significant).

• Observations of EEP from low-Earth orbiting satellites provide context for the

variation in the EEP levels: - either through observations of the “only just trapped” population

- or by trying to make measurements of the bounce loss cone

• While there is experimental evidence that relativistic electron precipitation is

important in the atmosphere, we struggle to measure it atall (lack of data).

• One of the best sources we have for this purpose are from the Polar

Operational Environmental Satellites (POES) .

BUT THESE DATA HAVE ISSUES.

• SPE cause overestimation of >30 keV electron fluxes by 103 or more. At these

times, or in the SAMA, the observations are almost certainly totally unusable.

• Low energy protons also affect the electron precipitation observations. But

there is an algorithm to correct for this as the effect is smallish (typically less

than a factor of 2).

• The geometry factor AND the energy fitting issue are still open questions and

are being worked on – they could easily lead to changes more than a factor of

10 (probably an underestimation of the EEP).

Craig standing in front of the Space Shuttle Discovery (OV-103) in the space hall of the Udvar Hazy Center (Smithsonian Air and Space Museum). He was visiting the Museum on the way to the VERSIM workshop in Brazil, and we had to spend most of a day around Dulles airport [31 August 2012].

Are there any questions?

Thankyou!

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Energetic (>10keV) and relativistic electron (>500keV ... · Energetic (>10keV) and relativistic electron (>500keV) precipitation into the mesosphere - evidence and limitations Craig