What is the belly pod on this Beech A200 Super King Air?
Short answer
It seems to be a US Army CEASAR pod, a communication intercepting and jamming system for electronic warfare, which has been flown in Iraq and Afghanistan. CEASAR is derived from AN/ALQ-227 EA used by US Navy on the EA-18G Growler.
The aircraft is equipped with missile detection and countermeasures. Two sensors are below the cockpit windscreen, two other on the aft fuselage. The chaff is released by a dispenser aft of the pod.
N40R is N46L's sister aircraft, and it also carries a CEASAR pod. The A200 variant used may be a C12-A or a C-12C (this is the model number visible on N40R fuselage). They are also known as Huron and HC-12.
High definition photo of N46R:

Photo by Keith Newsome on photobucket (source)
Detailed answer below.

About CEASAR
From Stars and Stripes:

The CEASAR, which stands for communications electronic attack with
surveillance and reconnaissance, is a rectangular metal box that holds
technology capable of both knocking out cell phones and radios and
intercepting transmissions, while an electronic warfare officer
onboard speaks directly to troops on the ground. It’s precise enough
to cover friendly soldiers while jamming insurgents, but not knocking
out communications for an entire village near a given operation.


From matthewaid.com:

Contractors Continue Flying CEASAR SIGINT Aircraft in Afghanistan
The CEASAR system mounted in these contractor-owned and
government-operated (COGO) aircraft is based on the Raytheon
AN/ALQ-227 EA system as fitted to the US Navy’s (USN’s) Boeing EA-18G
Growler electronic warfare (EW) aircraft. It is designed to provide
both a communications jamming and an intercept/monitoring capability.


CEASAR UAV replacement
Flying CEASAR on a modified A200 was already very much cheaper than using a pod on a fighter. However like for other functions, military are looking at using unmanned aircraft. The NERO pod has been designed for this purpose.
From Defense Systems:

NERO jams aboard a drone in Army test
The Army recently tested a jammer called the Networked Electronic
Warfare Remotely Operated, or NERO, attached to a Gray Eagle unmanned
combat aircraft at the Great Salt Lake Desert at Dugway Proving
Ground, Utah.



Missile Warning System AN/AAR-47
Closer view (N40R):

It detects infrared or radio sensing from a missile and commands a response, e.g. release of chaff or flare using the countermeasures dispensing system aft of the pod.
Two other sensors are located on the aft fuselage.
Sensor detail on another aircraft:

(source)

About the aircraft
From FAA, King Air A200, CN BC-1 thru BC-61 are variant C-12A used by US Army.
From JetPhotos.net this aircraft is used under call-sign Grizzly 23:

Registration: N46L
CN: BC-56
'Grizzly 23' taxies for departure to Wick.this is one of a number of
aircraft owned by Dynamic that operate on behalf of the US Army.

There are two C-12 aircraft equipped with CEASAR. The other is registered N40R, with call-sign Grizzly 12:

(source)
Huron are seconded or being replaced by Dash 8.