Earth’s magnetic field: Observing the Birkeland currents

Plots of AMPERE magnetic perturbations and radial current density from the northern hemisphere for 24 February 2014 with start times from 1530 UT through 1700 UT.

When the supersonic solar wind hits the Earth’s magnetic field, a powerful electrical connection occurs with Earth’s field, generating millions of amperes of current that drive the dazzling auroras.

These so-called Birkeland currents connect the ionosphere to the magnetosphere and channel solar wind energy to Earth’s uppermost atmosphere.

Solar storms release torrential blasts of solar wind that cause much stronger currents and can overload power grids and disrupt communications and navigation.

Now for the first time, scientists are making continuous, global measurements of the Birkeland currents, opening a new window on our understanding of our home planet’s response to solar storms.

Using the Active Magnetosphere and Planetary Electrodynamics Response Experiment, based on the 66 Iridium satellites orbiting the Earth, authors of a Geophysical Research Letters study have discovered that Earth’s response to onsets in forcing from the solar wind occurs in two distinct stages.

Currents first appear near noon in the polar regions and remain steady for about half an hour.

Then the second stage begins, when strong currents appear near midnight and eventually join the initial currents near noon.

Most of the solar wind energy is deposited in the polar atmosphere by processes initiated in the second stage.

The authors note that scientists are working to understand how the delay between the first and second stages could give near-term warning of impending space weather disruptions.

Full Citation: Anderson, B. J., H. Korth, C. L. Waters, D. L. Green, V. G. Merkin, R. J. Barnes, and L. P. Dyrud(2014), "Development of large-scale Birkeland currents determined from the Active Magnetosphere and Planetary Electrodynamics Response Experiment" - Geophys. Res. Lett., 41, 3017–3025, doi:10.1002/2014GL059941.

NRL SSULI: Satellite to measure ionosphere electron density

Says Andrew Nicholas, Principal NRL Investigator, "What we're looking at is naturally occurring air glow emissions from the upper atmosphere." 

Shown is the daily averaged electron density over two years; as a function of altitude and day of the year in the post-sunset, equatorial ionosphere. 

F18 SSULI measured the nightside oxygen 135.6 nm radiative recombination emission intensity, capturing the ionosphere's variations daily and over the long-term as associated with seasons and solar cycles. 

Credit: U.S. Naval Research Laboratory

On April 3rd, 2014, a satellite carrying a U.S. Naval Research Laboratory (NRL) space weather instrument will launch from Vandenberg Air Force Base.

Special Sensor Ultraviolet Limb Imager (SSULI)

Called the Special Sensor Ultraviolet Limb Imager (SSULI), "SSULI makes accurate measurements of the upper atmosphere and ionosphere that are ultimately useful to the warfighter," says Dr. Scott Budzien, the NRL Program Manager.

"Down in the lower atmosphere, the characteristics of weather that are important are temperature, precipitation, wind, and so on," he says.

"But in the ionosphere, the aspects that are important for our systems are electron density, the morphology and gradients of electron density, and the height where the ionosphere lies."

SSULI measures the density of the ionosphere (as ions or electrons per cubic centimeter). Says Andrew Nicholas, the Principal Investigator, "We are measuring naturally occurring airglow emissions from the upper atmosphere."

The density profiles go into the Department of Defense's weather system, which the U.S. Air Force has run since the 1940s to collect observations about terrestrial and space weather.

"SSULI helps provide a very good specification of the state of the atmosphere, a nowcast," says Budzien.

With a more accurate nowcast, forecasting models better predict space weather into the future.

Space weather is important for military operations, because how signals are transmitted or reflected influences the reliability of radar and of communication and navigation systems.

Configuration of Operational Polar Satellites

The April 3rd Defense Meteorological Satellite Program (DMSP) mission, Flight-19 (F19), is the fourth to carry a SSULI from NRL.

The DMSP satellites are low-earth orbiting satellites, flying at about 830-840 kilometers (km) above the ground.

"They fly in a sun-synchronous orbit, which means they are always at the same local time," says Budzien.

"The one we are launching in April is going into a terminator orbit, right at the day-night boundary."

NRL built five SSULIs in the early 1990s; the last launch is currently planned for 2016. But, says Budzien, "Based on the lessons learned from SSULI, we've developed the design for a smaller, more sensitive instrument."

Still just a concept, Budzien hopes to identify a sponsor who will help "to continue to provide improved products for the warfighter."

NASA CINDI Mission: Monitoring the Ionosphere

When high frequency radio waves, such as those used for the Global Positioning System (GPS), travel through a disturbed layer of Earth’s electrically charged atmosphere, the ionosphere, they can be disrupted. 

Credit: U.S. Air Force Research Laboratory (AFRL)

On April 16, 2008, a suite of NASA instruments was launched into space to study a unique region of Earth’s upper atmosphere: the electrically charged region called the ionosphere.

The instruments, known collectively as CINDI (Coupled Ion-Neutral Dynamics Investigation), fly aboard an Air Force Research Laboratory satellite called C/NOFS (Communications/Navigation Outage Forecasting System) to study this region that hovers some 60 to 400 miles above Earth.

The ionosphere is crucial for modern communications. Low-frequency radio waves bounce off it to travel from one part of Earth to another.

Various satellites, including the global positioning system (GPS), send high-frequency radio waves through the ionosphere down to receivers on Earth. In this region the right conditions exist to allow incoming energy from the sun to knock electrons off the atoms.

So the area seethes with charged particles moving under forces of both conventional winds and of electric fields that drive the particles perpendicular to the magnetic field lines surrounding Earth.

The ionosphere changes constantly: between night and day, with the seasons, between the equator and the poles, and with every incoming burst of radiation from the sun.

These simulations of the nighttime, low latitude ionosphere – representing altitudes of about 120 to 750 miles above Earth -- show how "chimneys" of lower density form at the base of the ionosphere and then rise up, creating branches at ever smaller scales. 

These perturbations can disturb radio waves moving through the region. 

Each colour shows a different density of material. 

The colour green corresponds to low-density regions. 

Red represents the densest region of the ionosphere, over 100 times more dense. 

Credit: John Retterer

Small changes in the ionosphere, at night, for example, can simply garble the radio waves – a phenomenon known as scintillation. But at worst, an event such as a giant solar flare can black out radio transmissions completely.

Rob Pfaff

“All the space assets we have come to rely on, in one way or another propagate radio waves through this region of Earth’s atmosphere so we need to understand it better,” says Rob Pfaff, project scientist for CINDI at NASA’s Goddard Space Flight Center in Greenbelt, Md.

“Our goals with CINDI are to determine why the region becomes irregular, and, ultimately, to be able to forecast when and where it will be irregular.”

Studying the ionosphere is like trying to understand a very complicated lava lamp, in which blobs of different materials move up and down in response to changes in temperature.

In the case of the ionosphere, scientists want to understand the kinds of heat and energy affecting particle movement and how these motions interact and rely on each other.

Every piece of data, such as where certain particles appear and what causes areas of lower and higher density, represents a significant advance toward predicting change in the region.

Over the last five years, CINDI has gleaned information about the distribution in height of different kinds of particles, about how winds sweep through the atmosphere in response to events on the sun, and what causes density changes in both the charged and neutral particles in the ionosphere.

One of the early observations by CINDI was of the top of the ionosphere layer, which is dominated by hydrogen ions near dawn.

The middle layer of the area is dominated by oxygen ions. In 2008, CINDI found that the transition region, where there is an equal number of both particles, was located about 370 miles up, much closer to Earth than expected.

Since CINDI launched at a time of low solar activity -- a period of the sun’s approximate 11-year cycle known as solar minimum – the mission has had the chance to observe how the ionosphere changes as the sun’s activity ramps up to solar maximum, currently expected in late 2013.

Over five years of watching, this oxygen/hydrogen transition region has now moved up in space to over 430 miles in altitude, providing an indicator of how Earth’s atmosphere swells and expands in response to increased energy coming in from the sun.